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【leetcode刷题笔记】4Sum

2024-07-13 23:30:26   220人阅读

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 }


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