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【leetcode刷题笔记】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)

题解：参考3Sum，把4Sum问题转换成3Sum，再转换成2Sum，如下图所示：

代码如下：

 1 public class Solution {
 2     public List<List<Integer>> fourSum(int[] num, int target) {
 3         List<List<Integer>> answer = new ArrayList<List<Integer>>();
 4         if(num == null || num.length == 0)
 5             return answer;
 6         
 7         Arrays.sort(num);
 8         int length = num.length;
 9         for(int i = 0;i < length;i++){
10             if(i!=0 && num[i]== num[i-1] )
11                 continue;
12             for(int j = i+1;j < length;j++ ){
13                 if(j != i+1 && num[j] == num[j-1] )
14                     continue;
15                 int start = j + 1;
16                 int last = length - 1;
17                 while(start < last){
18                     int sum = num[i]+num[j]+num[start]+num[last];
19                     if(sum == target){
20                         ArrayList<Integer> result = new ArrayList<Integer>();
21                         result.add(num[i]);
22                         result.add(num[j]);
23                         result.add(num[start]);
24                         result.add(num[last]);
25                         answer.add(result);
26                         start++;
27                         last--;
28                         while(start < last && num[start-1] == num[start])
29                             start++;
30                     }
31                     else if(sum < target){
32                         start++;
33                     }
34                     else {
35                         last--;
36                     }
37                 }
38             }
39         }
40         return answer;
41     }
42 }