leetcode--4Sum

Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.

Note:

• Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)
• The solution set must not contain duplicate quadruplets.

    For example, given array S = {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)

[java]  view plain  copy

1. public class Solution {  
2.     public List<List<Integer>> fourSum(int[] nums, int target) {  
3.         List<List<Integer>> result = new ArrayList<List<Integer>>();    
4.         Arrays.sort(nums);  
5.         for(int i=0;i<nums.length;i++){    
6.             if(i!=0 && nums[i]==nums[i-1]){//查重  
7.                 continue;  
8.             }  
9.             for(int j=i+1;j<nums.length;j++){  
10.                 if(j!=i+1 && nums[j]==nums[j-1]){//查重  
11.                     continue;  
12.                 }  
13.                  int low = j+1;    
14.                  int high = nums.length-1;                            
15.                  while(low<high){          
16.                      int sum = nums[i]+nums[j]+nums[low]+nums[high];  
17.                      if(sum>target){  
18.                          high--;  
19.                      }else if(sum < target){  
20.                          low++;  
21.                      }else if(low!=j+1&&nums[low]==nums[low-1]){//查重  
22.                          low++;  
23.                      }else if(high!=nums.length-1&&nums[high]==nums[high+1]){//查重  
24.                          high--;  
25.                      }else{  
26.                          List<Integer> list = new ArrayList<Integer>();     
27.                          list.add(nums[i]);list.add(nums[j]);  
28.                          list.add(nums[low]);list.add(nums[high]);    
29.                          result.add(list);                            
30.                          low++;    
31.                          high--;    
32.                      }  
33.                  }                       
34.             }                 
35.         }    
36.         
37.         return result;    
38.     }  
39. }  

原文链接http://blog.youkuaiyun.com/crazy__chen/article/details/45567371