18. 4Sum
Given an array nums of n integers and an integer target, are there elements a, b, c, and d in nums such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.
Note:
The solution set must not contain duplicate quadruplets.
Example:
Given array nums = [1, 0, -1, 0, -2, 2], and target = 0.A solution set is:[ ?[-1, ?0, 0, 1], ?[-2, -1, 1, 2], ?[-2, ?0, 0, 2]]
题意：给定一个数字数组和一个目标值，找出所有4项组合使得四项相加等于目标值
代码如下：
var fourSum = function(nums, target) { ???var len=nums.length; ???var res=[]; ???if(nums.length<4 || !nums) return res; ???if(len===4 && nums[0]+nums[1]+nums[2]+nums[3]==target) return [nums]; ???//排序 ???nums=nums.sort(function(a,b){return a-b;}) ???????for(var i=0;i<len-3;i++){ ???????//判断相等项 ???????// if(nums[i]===nums[i-1]) continue; ???????for(var j=i+1;j<len-2;j++){ ???????????if(j>i+1 && nums[j]===nums[j-1]) continue; ???????????var start=j+1,end=len-1; ???????????while(start<end){ ???????????????var sum=nums[i]+nums[j]+nums[start]+nums[end]; ???????????????if(sum===target){ ??????????????????????res.push([nums[i],nums[j],nums[start],nums[end]]) ?????????????????????++start; ???????????????????--end; ???????????????}else if(nums[i]+nums[j]+nums[start]+nums[end]>target){ ???????????????????--end; ???????????????}else{ ???????????????????++start; ???????????????} ???????????} ???????} ???} ??//去重 ???var hash = {}; ?????var result = []; ?????res.forEach(item=>{ ???????if(!hash[item]){ ???????????result.push(item); ???????????hash[item]=true; ???????} ???}) ???return result;};18. 4Sum（js）
原文地址：https://www.cnblogs.com/xingguozhiming/p/10387344.html