18. 4Sum（双指针）

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]
]

 1 class Solution {
 2     public List<List<Integer>> fourSum(int[] nums, int target) {
 3         List<List<Integer>> res = new LinkedList<>();
 4         if (nums.length<4) return res;
 5         Arrays.sort(nums);
 6         for(int i=0;i<nums.length-3;i++){
 7             if(i>0&&nums[i]==nums[i-1]) continue;
 8             for(int j=i+1;j<nums.length-2;j++){
 9                 if(j>i+1&&nums[j]==nums[j-1]) continue;
10
11                 int lo = j+1,hi = nums.length-1;
12                 while(lo<hi){
13                     int sum = nums[i]+nums[j]+nums[lo]+nums[hi];
14                     if(sum==target){
15                         res.add(Arrays.asList(nums[i],nums[j],nums[lo],nums[hi]));
16
17                         //答案去重
18                         while(lo<hi&&nums[lo]==nums[lo+1])  lo++;
19                         while(lo<hi&&nums[hi]==nums[hi-1])  hi--;
20
21                         lo++;
22                         hi--;
23                     }
24
25                     else if(sum<target) lo++;
26                     else hi--;
27                 }
28
29             }
30         }
31         return res;
32     }
33 }

原文地址：https://www.cnblogs.com/zle1992/p/9314157.html