期刊文献+
共找到984篇文章
< 1 2 50 >
每页显示 20 50 100
Operations of Knapsack Sprayer and Its Impacts on Physiological Parameters of Selected Operators
1
作者 Bukola Olanrewaju Afolabi Akinfoye Oyime Daniel Adejumo +1 位作者 Ayoade Oladele Atere Titus Adeyinka Ilori 《Engineering(科研)》 CAS 2023年第2期59-67,共9页
The study was aimed at determining the impacts of operating Manually Operated Hand Lever Knapsack Sprayers (MOHLKS) on physiological responses of the operators as dependent on anthropometric variations and sex. Twenty... The study was aimed at determining the impacts of operating Manually Operated Hand Lever Knapsack Sprayers (MOHLKS) on physiological responses of the operators as dependent on anthropometric variations and sex. Twenty eight subjects, (4 female, 24 male) Mean ± SD: Age 22.5 ± 1.92, 24.29 ± 2.2 years;Body Mass Index 24.6 ± 4.8, 21.7 ± 2.4 kg/m<sup>2</sup> were employed in the study. Selected anthropometric parameters of weight and height were used to determine body mass index (BMI), with these are arm-reach forward, elbow to fingertip, hand length and hand width were measured to establish human variations in diversity. Subjects undertook the operation at 5 replicates each, before and after which information about operators’ body pain locations and body physiological changes of heart rates were obtained. Measured parameters were used in the determination of expended energy (EE), physiological cost (PC), oxygen intake (VO<sub>2</sub>) and aerobic power (VO<sub>2</sub>max). Alongside with these were operational parameters of stroke, pace and time taken to get the operation done and environmental factors of temperature and relative humidity. The results revealed on the average that the BMI (24.61 ± 4.78 kg/m<sup>2</sup>) in female operators was higher, this corresponded to PC and VO<sub>2</sub>, while the VO<sub>2</sub>max (34.83 ± 3.30 ml/min/kg) in males is higher. More EE was obtained in female subjects (3.53 ± 3.76 kCal/min) as compared to male subjects (3.42 ± 7.48 kCal/min). The main effects plot of operational factors on EE displayed the stroke made by the subjects during spraying operation as parameter with largest effect on EE. Regression equation for EE and PCI is given as PCI = 1.97 + 25.2 EE, while the P-value at α = 0.05 is 0.000 and R<sup>2</sup> = 98.8%. Post operational body pain showed that 19 out of 28 subjects incurred at least one type of body pain, with shoulder pain as most frequent. The results of the study suggest that early incidence of fatigue may occur in female operators as compared to the males, and in addition, cumulative trauma at shoulder, back, and upper and lower arm may result over time. Hence, it is recommended that the tank volume should be reduced and the straps for the shoulders should be supported with additional cushion. 展开更多
关键词 Cumulativetrauma knapsack Sprayer Operational Effect Physiological Changes STROKE
下载PDF
NP破译的Knapsack体制 被引量:2
2
作者 曹珍富 刘锐 《高校应用数学学报(A辑)》 CSCD 北大核心 1989年第1期1-5,共5页
本文提出一种新的Knapsack体制,其安全性将基于线性问题——一个非常困难问题的求解。同时指出通过(W,M)变换,新体制的安全性将主要依赖于矩阵复盖的NP——完全性问题。
关键词 knapsack体制 NP破译 NP完全问题
下载PDF
Knapsacks约束的弧相容改进算法
3
作者 黄蔚 付兴宇 李占山 《吉林大学学报(理学版)》 CAS CSCD 北大核心 2017年第1期95-102,共8页
通过修改背包约束弧相容算法的数据结构,将点阵图改为有向图,解决了原背包约束弧相容算法中存在冗余计算和无效操作的问题,加快了算法对问题的求解效率.对比实验结果表明:在面对同一类问题时,因为数据结构更复杂,改进算法的初始化时间... 通过修改背包约束弧相容算法的数据结构,将点阵图改为有向图,解决了原背包约束弧相容算法中存在冗余计算和无效操作的问题,加快了算法对问题的求解效率.对比实验结果表明:在面对同一类问题时,因为数据结构更复杂,改进算法的初始化时间虽增加,但求解时间提高了20%~50%;在面对求解难度较高的问题时,改进算法能更好地缩减求解问题的时间. 展开更多
关键词 约束满足问题 弧相容 knapsacks约束
下载PDF
Gompertz PSO variants for Knapsack and Multi-Knapsack Problems
4
作者 Pinkey Chauhan Millie Pant Kusum Deep 《Applied Mathematics(A Journal of Chinese Universities)》 SCIE CSCD 2021年第4期611-630,共20页
Particle Swarm Optimization,a potential swarm intelligence heuristic,has been recognized as a global optimizer for solving various continuous as well as discrete optimization problems.Encourged by the performance of G... Particle Swarm Optimization,a potential swarm intelligence heuristic,has been recognized as a global optimizer for solving various continuous as well as discrete optimization problems.Encourged by the performance of Gompertz PSO on a set of continuous problems,this works extends the application of Gompertz PSO for solving binary optimization problems.Moreover,a new chaotic variant of Gompertz PSO namely Chaotic Gompertz Binary Particle Swarm Optimization(CGBPSO)has also been proposed.The new variant is further analysed for solving binary optimization problems.The new chaotic variant embeds the notion of chaos into GBPSO in later stages of searching process to avoid stagnation phenomena.The efficiency of both the Binary PSO variants has been tested on different sets of Knapsack Problems(KPs):0-1 Knapsack Problem(0-1 KP)and Multidimensional Knapsack Problems(MKP).The concluding remarks have made on the basis of detailed analysis of results,which comprises the comparison of results for Knapsack and Multidimensional Knapsack problems obtained using BPSO,GBPSO and CGBPSO. 展开更多
关键词 Binary PSO knapsack Problems Multi knapsack Problems Gompertz function CHAOS
下载PDF
对矩阵覆盖掩护下的Knapsack体制的讨论 被引量:3
5
作者 曹珍富 郑宝东 《哈尔滨工业大学学报》 EI CAS CSCD 北大核心 1990年第6期34-41,共8页
本文对两类矩阵覆盖(MC)掩护下的Knapsack体制进行了讨论,指出这两类体制的不安全因素与某些表达错误。同时,提出了两类修改的MC-Knapsack体制。修改后的体制与原先的两类Knapsack体制比较,具有良好的密码学特性,例如减小了密钥量、提... 本文对两类矩阵覆盖(MC)掩护下的Knapsack体制进行了讨论,指出这两类体制的不安全因素与某些表达错误。同时,提出了两类修改的MC-Knapsack体制。修改后的体制与原先的两类Knapsack体制比较,具有良好的密码学特性,例如减小了密钥量、提高了安全性等。 展开更多
关键词 矩阵覆盖体制 背包 密码
下载PDF
SOME EXTENDED KNAPSACK PROBLEMS INVOLVING JOB PARTITION BETWEEN TWO PARTIES 被引量:8
6
作者 Gu Yanhong Chen Quanle 《Applied Mathematics(A Journal of Chinese Universities)》 SCIE CSCD 2007年第3期366-370,共5页
Some novel applications and pragmatic variations of knapsack problem (KP) are presented and constructed, which are formulated and developed from a model initiated in this paper on profit allocation from partition of... Some novel applications and pragmatic variations of knapsack problem (KP) are presented and constructed, which are formulated and developed from a model initiated in this paper on profit allocation from partition of jobs in terms of two-person discrete cooperation game. 展开更多
关键词 knapsack problem profit allocation job partition.
下载PDF
A branch-and-bound algorithm for multi-dimensional quadratic 0-1 knapsack problems 被引量:2
7
作者 孙娟 盛红波 孙小玲 《Journal of Shanghai University(English Edition)》 CAS 2007年第3期233-236,共4页
In this paper, a branch-and-bound method for solving multi-dimensional quadratic 0-1 knapsack problems was studied. The method was based on the Lagrangian relaxation and the surrogate constraint technique for finding ... In this paper, a branch-and-bound method for solving multi-dimensional quadratic 0-1 knapsack problems was studied. The method was based on the Lagrangian relaxation and the surrogate constraint technique for finding feasible solutions. The Lagrangian relaxations were solved with the maximum-flow algorithm and the Lagrangian bounds was determined with the outer approximation method. Computational results show the efficiency of the proposed method for multi-dimensional quadratic 0-1 knapsack problems. 展开更多
关键词 multi-dimensional quadratic 0-1 knapsack problem branch-and-bound method Lagrangian relaxation outer approximation surrogate constraint.
下载PDF
Quantum-inspired ant algorithm for knapsack problems 被引量:3
8
作者 Wang Honggang Ma Liang Zhang Huizhen Li Gaoya 《Journal of Systems Engineering and Electronics》 SCIE EI CSCD 2009年第5期1012-1016,共5页
The knapsack problem is a well-known combinatorial optimization problem which has been proved to be NP-hard. This paper proposes a new algorithm called quantum-inspired ant algorithm (QAA) to solve the knapsack prob... The knapsack problem is a well-known combinatorial optimization problem which has been proved to be NP-hard. This paper proposes a new algorithm called quantum-inspired ant algorithm (QAA) to solve the knapsack problem. QAA takes the advantage of the principles in quantum computing, such as qubit, quantum gate, and quantum superposition of states, to get more probabilistic-based status with small colonies. By updating the pheromone in the ant algorithm and rotating the quantum gate, the algorithm can finally reach the optimal solution. The detailed steps to use QAA are presented, and by solving series of test cases of classical knapsack problems, the effectiveness and generality of the new algorithm are validated. 展开更多
关键词 knapsack problem quantum computing ant algorithm quantum-inspired ant algorithm.
下载PDF
An efficient algorithm for multi-dimensional nonlinear knapsack problems 被引量:1
9
作者 陈娟 孙小玲 郭慧娟 《Journal of Shanghai University(English Edition)》 CAS 2006年第5期393-398,共6页
Multi-dimensional nonlinear knapsack problem is a bounded nonlinear integer programming problem that maximizes a separable nondecreasing function subject to multiple separable nondecreasing constraints. This problem i... Multi-dimensional nonlinear knapsack problem is a bounded nonlinear integer programming problem that maximizes a separable nondecreasing function subject to multiple separable nondecreasing constraints. This problem is often encountered in resource allocation, industrial planning and computer network. In this paper, a new convergent Lagrangian dual method was proposed for solving this problem. Cutting plane method was used to solve the dual problem and to compute the Lagrangian bounds of the primal problem. In order to eliminate the duality gap and thus to guarantee the convergence of the algorithm, domain cut technique was employed to remove certain integer boxes and partition the revised domain to a union of integer boxes. Extensive computational results show that the proposed method is efficient for solving large-scale multi-dimensional nonlinear knapsack problems. Our numerical results also indicate that the cutting plane method significantly outperforms the subgradient method as a dual search procedure. 展开更多
关键词 nonlinear integer programming nonlinear knapsack problem Lagrangian relaxation cutting plane subgradient method.
下载PDF
A Hybrid Parallel Multi-Objective Genetic Algorithm for 0/1 Knapsack Problem 被引量:3
10
作者 Sudhir B. Jagtap Subhendu Kumar Pani Ganeshchandra Shinde 《Journal of Software Engineering and Applications》 2011年第5期316-319,共4页
In this paper a hybrid parallel multi-objective genetic algorithm is proposed for solving 0/1 knapsack problem. Multi-objective problems with non-convex and discrete Pareto front can take enormous computation time to ... In this paper a hybrid parallel multi-objective genetic algorithm is proposed for solving 0/1 knapsack problem. Multi-objective problems with non-convex and discrete Pareto front can take enormous computation time to converge to the true Pareto front. Hence, the classical multi-objective genetic algorithms (MOGAs) (i.e., non- Parallel MOGAs) may fail to solve such intractable problem in a reasonable amount of time. The proposed hybrid model will combine the best attribute of island and Jakobovic master slave models. We conduct an extensive experimental study in a multi-core system by varying the different size of processors and the result is compared with basic parallel model i.e., master-slave model which is used to parallelize NSGA-II. The experimental results confirm that the hybrid model is showing a clear edge over master-slave model in terms of processing time and approximation to the true Pareto front. 展开更多
关键词 Multi-Objective Genetic Algorithm PARALLEL Processing Techniques NSGA-II 0/1 knapsack Problem TRIGGER MODEL CONE Separation MODEL Island MODEL
下载PDF
A Weight-Coded Evolutionary Algorithm for the Multidimensional Knapsack Problem 被引量:2
11
作者 Quan Yuan Zhixin Yang 《Advances in Pure Mathematics》 2016年第10期659-675,共17页
A revised weight-coded evolutionary algorithm (RWCEA) is proposed for solving multidimensional knapsack problems. This RWCEA uses a new decoding method and incorporates a heuristic method in initialization. Computatio... A revised weight-coded evolutionary algorithm (RWCEA) is proposed for solving multidimensional knapsack problems. This RWCEA uses a new decoding method and incorporates a heuristic method in initialization. Computational results show that the RWCEA performs better than a weight-coded evolutionary algorithm pro-posed by Raidl (1999) and to some existing benchmarks, it can yield better results than the ones reported in the OR-library. 展开更多
关键词 Weight-Coding Evolutionary Algorithm Multidimensional knapsack Problem (MKP)
下载PDF
SEMI-DEFINITE RELAXATION ALGORITHM OF MULTIPLE KNAPSACK PROBLEM
12
作者 Chen Feng Yao EnyuDept.ofMath.,ZhejiangUniv.,Hangzhou310027,China 《Applied Mathematics(A Journal of Chinese Universities)》 SCIE CSCD 2002年第2期241-250,共10页
The multiple knapsack problem denoted by MKP (B,S,m,n) can be defined as fol- lows.A set B of n items and a set Sof m knapsacks are given such thateach item j has a profit pjand weightwj,and each knapsack i has a ca... The multiple knapsack problem denoted by MKP (B,S,m,n) can be defined as fol- lows.A set B of n items and a set Sof m knapsacks are given such thateach item j has a profit pjand weightwj,and each knapsack i has a capacity Ci.The goal is to find a subset of items of maximum profit such that they have a feasible packing in the knapsacks.MKP(B,S,m,n) is strongly NP- Complete and no polynomial- time approximation algorithm can have an approxima- tion ratio better than0 .5 .In the last ten years,semi- definite programming has been empolyed to solve some combinatorial problems successfully.This paper firstly presents a semi- definite re- laxation algorithm (MKPS) for MKP (B,S,m,n) .It is proved that MKPS have a approxima- tion ratio better than 0 .5 for a subclass of MKP (B,S,m,n) with n≤ 1 0 0 ,m≤ 5 and maxnj=1{ wj} minmi=1{ Ci} ≤ 2 3 . 展开更多
关键词 multiple knapsack problem semi- definite relaxation approximation algorithm combina- torial optimization.
下载PDF
Uncertain bilevel knapsack problem and its solution
13
作者 Junjie Xue Ying Wang Jiyang Xiao 《Journal of Systems Engineering and Electronics》 SCIE EI CSCD 2017年第4期717-724,共8页
This paper aims at providing an uncertain bilevel knapsack problem (UBKP) model, which is a type of BKPs involving uncertain variables. And then an uncertain solution for the UBKP is proposed by defining PE Nash equil... This paper aims at providing an uncertain bilevel knapsack problem (UBKP) model, which is a type of BKPs involving uncertain variables. And then an uncertain solution for the UBKP is proposed by defining PE Nash equilibrium and PE Stackelberg Nash equilibrium. In order to improve the computational efficiency of the uncertain solution, several operators (binary coding distance, inversion operator, explosion operator and binary back learning operator) are applied to the basic fireworks algorithm to design the binary backward fireworks algorithm (BBFWA), which has a good performance in solving the BKP. As an illustration, a case study of the UBKP model and the P-E uncertain solution is applied to an armaments transportation problem. 展开更多
关键词 UNCERTAINTY bilevel programming knapsack problem binary backward fireworks algorithm
下载PDF
Surrogate dual method for multi-dimensional nonlinear knapsack problems
14
作者 孔珊珊 孙小玲 《Journal of Shanghai University(English Edition)》 CAS 2007年第4期340-343,共4页
Multi-dimensional nonlinear knapsack problems are often encountered in resource allocation, industrial planning and computer networks. In this paper, a surrogate dual method was proposed for solving this class of prob... Multi-dimensional nonlinear knapsack problems are often encountered in resource allocation, industrial planning and computer networks. In this paper, a surrogate dual method was proposed for solving this class of problems. Multiply constrained problem was relaxed to a singly constrained problem by using the surrogate technique. To compute tighter bounds of the primal problem, the cutting plane method was used to solve the surrogate dual problem, where the surrogate relaxation problem was solved by the 0-1 linearization method. The domain cut technique was employed to eliminate the duality gap and thus to guarantee the convergence of tile algorithm. Numerical results were reported for large-scale multi-dimensional nonlinear knapsack problems. 展开更多
关键词 nonlinear knapsack problem surrogate dual Lagrangian dual domain cut
下载PDF
A Combinatorial Optimized Knapsack Linear Space for Information Retrieval
15
作者 Varghese S.Chooralil Vinodh P.Vijayan +3 位作者 Biju Paul M.M.Anishin Raj B.Karthikeyan G.Manikandan 《Computers, Materials & Continua》 SCIE EI 2021年第3期2891-2903,共13页
Key information extraction can reduce the dimensional effects while evaluating the correct preferences of users during semantic data analysis.Currently,the classifiers are used to maximize the performance of web-page ... Key information extraction can reduce the dimensional effects while evaluating the correct preferences of users during semantic data analysis.Currently,the classifiers are used to maximize the performance of web-page recommendation in terms of precision and satisfaction.The recent method disambiguates contextual sentiment using conceptual prediction with robustness,however the conceptual prediction method is not able to yield the optimal solution.Context-dependent terms are primarily evaluated by constructing linear space of context features,presuming that if the terms come together in certain consumerrelated reviews,they are semantically reliant.Moreover,the more frequently they coexist,the greater the semantic dependency is.However,the influence of the terms that coexist with each other can be part of the frequency of the terms of their semantic dependence,as they are non-integrative and their individual meaning cannot be derived.In this work,we consider the strength of a term and the influence of a term as a combinatorial optimization,called Combinatorial Optimized Linear Space Knapsack for Information Retrieval(COLSK-IR).The COLSK-IR is considered as a knapsack problem with the total weight being the“term influence”or“influence of term”and the total value being the“term frequency”or“frequency of term”for semantic data analysis.The method,by which the term influence and the term frequency are considered to identify the optimal solutions,is called combinatorial optimizations.Thus,we choose the knapsack for performing an integer programming problem and perform multiple experiments using the linear space through combinatorial optimization to identify the possible optimum solutions.It is evident from our experimental results that the COLSK-IR provides better results than previous methods to detect strongly dependent snippets with minimum ambiguity that are related to inter-sentential context during semantic data analysis. 展开更多
关键词 Key information extraction web-page context-dependent nonintegrative combinatorial optimization knapsack
下载PDF
Improved Parallel Three-List Algorithm for the Knapsack Problem without Memory Conflicts
16
作者 潘军 李肯立 李庆华 《Journal of Southwest Jiaotong University(English Edition)》 2006年第1期7-14,共8页
Based on the two-list algorithm and the parallel three-list algorithm, an improved parallel three-list algorithm for knapsack problem is proposed, in which the method of divide and conquer, and parallel merging withou... Based on the two-list algorithm and the parallel three-list algorithm, an improved parallel three-list algorithm for knapsack problem is proposed, in which the method of divide and conquer, and parallel merging without memory conflicts are adopted. To find a solution for the n-element knapsack problem, the proposed algorithm needs O(2^3n/8) time when O(2^3n/8) shared memory units and O(2^n/4) processors are available. The comparisons between the proposed algorithm and 10 existing algorithms show that the improved parallel three-fist algorithm is the first exclusive-read exclusive-write (EREW) parallel algorithm that can solve the knapsack instances in less than O(2^n/2) time when the available hardware resource is smaller than O(2^n/2) , and hence is an improved result over the past researches. 展开更多
关键词 knapsack problem NP-HARD Parallel algorithm Memory conflicts Hardware-time tradeoff
下载PDF
An Optimal Parallel Algorithm for the Knapsack Problem Based on EREW
17
作者 李肯立 蒋盛益 +1 位作者 王卉 李庆华 《Journal of Southwest Jiaotong University(English Edition)》 2003年第2期131-137,共7页
A new parallel algorithm is proposed for the knapsack problem where the method of divide and conquer is adopted. Based on an EREW-SIMD machine with shared memory, the proposed algorithm utilizes O(2 n/4 ) 1-ε ... A new parallel algorithm is proposed for the knapsack problem where the method of divide and conquer is adopted. Based on an EREW-SIMD machine with shared memory, the proposed algorithm utilizes O(2 n/4 ) 1-ε processors, 0≤ ε ≤1, and O(2 n/2 ) memory to find a solution for the n -element knapsack problem in time O(2 n/4 (2 n/4 ) ε) . The cost of the proposed parallel algorithm is O(2 n/2 ) , which is an optimal method for solving the knapsack problem without memory conflicts and an improved result over the past researches. 展开更多
关键词 knapsack problem NP-COMPLETE parallel algorithm divide and conquer
下载PDF
On Merging Cover Inequalities for Multiple Knapsack Problems
18
作者 Randal Hickman Todd Easton 《Open Journal of Optimization》 2015年第4期141-155,共15页
This paper describes methods to merge two cover inequalities and also simultaneously merge multiple cover inequalities in a multiple knapsack instance. Theoretical results provide conditions under which merged cover i... This paper describes methods to merge two cover inequalities and also simultaneously merge multiple cover inequalities in a multiple knapsack instance. Theoretical results provide conditions under which merged cover inequalities are valid. Polynomial time algorithms are created to find merged cover inequalities. A computational study demonstrates that merged inequalities improve the solution times for benchmark multiple knapsack instances by about 9% on average over CPLEX with default settings. 展开更多
关键词 Multiple knapsack Problem Cutting Plane COVER INEQUALITY INEQUALITY MERGING Pseudocost INTEGER Programming
下载PDF
An Algorithm of 0-1 Knapsack Problem Based on Economic Model
19
作者 Yingying Tian Jianhui Lv Liang Zheng 《Journal of Applied Mathematics and Physics》 2013年第4期31-35,共5页
In order to optimize the knapsack problem further, this paper proposes an innovative model based on dynamic expectation efficiency, and establishes a new optimization algorithm of 0-1 knapsack problem after analysis a... In order to optimize the knapsack problem further, this paper proposes an innovative model based on dynamic expectation efficiency, and establishes a new optimization algorithm of 0-1 knapsack problem after analysis and research. Through analyzing the study of 30 groups of 0-1 knapsack problem from discrete coefficient of the data, we can find that dynamic expectation model can solve the following two types of knapsack problem. Compared to artificial glowworm swam algorithm, the convergence speed of this algorithm is ten times as fast as that of artificial glowworm swam algorithm, and the storage space of this algorithm is one quarter that of artificial glowworm swam algorithm. To sum up, it can be widely used in practical problems. 展开更多
关键词 0-1 knapsack ECONOMIC Model Optimization ALGORITHM STORAGE SPACE
下载PDF
Big Data Flow Adjustment Using Knapsack Problem
20
作者 Eyman Yosef Ahmed Salama M. Elsayed Wahed 《Journal of Computer and Communications》 2018年第10期30-39,共10页
The advancements of mobile devices, public networks and the Internet of creature huge amounts of complex data, both construct & unstructured are being captured in trust to allow organizations to produce better bus... The advancements of mobile devices, public networks and the Internet of creature huge amounts of complex data, both construct & unstructured are being captured in trust to allow organizations to produce better business decisions as data is now pivotal for an organizations success. These enormous amounts of data are referred to as Big Data, which enables a competitive advantage over rivals when processed and analyzed appropriately. However Big Data Analytics has a few concerns including Management of Data, Privacy & Security, getting optimal path for transport data, and Data Representation. However, the structure of network does not completely match transportation demand, i.e., there still exist a few bottlenecks in the network. This paper presents a new approach to get the optimal path of valuable data movement through a given network based on the knapsack problem. This paper will give value for each piece of data, it depends on the importance of this data (each piece of data defined by two arguments size and value), and the approach tries to find the optimal path from source to destination, a mathematical models are developed to adjust data flows between their shortest paths based on the 0 - 1 knapsack problem. We also take out computational experience using the commercial software Gurobi and a greedy algorithm (GA), respectively. The outcome indicates that the suggest models are active and workable. This paper introduced two different algorithms to study the shortest path problems: the first algorithm studies the shortest path problems when stochastic activates and activities does not depend on weights. The second algorithm studies the shortest path problems depends on weights. 展开更多
关键词 0 - 1 knapsack Problem BIG DATA BIG DATA ANALYTICS BIG DAO TA Inconsistencies
下载PDF
上一页 1 2 50 下一页 到第
使用帮助 返回顶部