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)

Discuss

注意，其实与3Sum差不多，只不过是多了一重循环

public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {ArrayList<ArrayList<Integer>> arr=new ArrayList<ArrayList<Integer>>();Arrays.sort(num);for(int i=0;i<num.length;i++){if(i>0&&(num[i]==num[i-1]))continue;for(int j=i+1;j<num.length;j++){if(j>i+1&&(num[j]==num[j-1]))continue;int k=j+1;int t=num.length-1;while(k<t){if(k>j+1&&(num[k]==num[k-1])){k++;continue;}if(t<num.length-1&&(num[t]==num[t+1])){t--;continue;}if(num[i]+num[j]+num[k]+num[t]==target){ArrayList<Integer> tempArr=new ArrayList<Integer>();tempArr.add(num[i]);tempArr.add(num[j]);tempArr.add(num[k]);tempArr.add(num[t]);arr.add(tempArr);k++;}else if(num[i]+num[j]+num[k]+num[t]>target){t--;}else if(num[i]+num[j]+num[k]+num[t]<target){k++;}}}}return arr;    }