【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：

 public class Solution {

     public List<List<Integer>> fourSum(int[] num, int target) {

         List<List<Integer>> answer = new ArrayList<List<Integer>>();

         if(num == null || num.length == 0)

             return answer;

         Arrays.sort(num);

         int length = num.length;

         for(int i = 0;i < length;i++){

             if(i!=0 && num[i]== num[i-1] )

                 continue;

             for(int j = i+1;j < length;j++ ){

                 if(j != i+1 && num[j] == num[j-1] )

                     continue;

                 int start = j + 1;

                 int last = length - 1;

                 while(start < last){

                     int sum = num[i]+num[j]+num[start]+num[last];

                     if(sum == target){

                         ArrayList<Integer> result = new ArrayList<Integer>();

                         result.add(num[i]);

                         result.add(num[j]);

                         result.add(num[start]);

                         result.add(num[last]);

                         answer.add(result);

                         start++;

                         last--;

                         while(start < last && num[start-1] == num[start])

                             start++;

                     }

                     else if(sum < target){

                         start++;

                     }

                     else {

                         last--;

                     }

                 }

             }

         }

         return answer;

     }

 }

巴特西

【leetcode刷题笔记】4Sum

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