解题（Solution -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)
代码如下：

import java.util.*;
public class Solution {
    public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
        ArrayList<ArrayList<Integer>> all=new ArrayList<ArrayList<Integer>>();
        Arrays.sort(num);
        for(int i=0;i<num.length-3;i++){
            if(num[i]+num[i+1]+num[i+2]+num[i+3]>target){
                break;
            }
            if(num[i]+num[num.length-1]+num[num.length-2]+num[num.length-3]<target){
                continue;
            }
            int result=target-num[i];
                for(int j=i+1;j<num.length-2;j++){
                        if(num[i]+num[j]+num[j+1]+num[j+2]>target){
                            break;
                        }
                        if(num[i]+num[num.length-1]+num[num.length-2]+num[j]<target){
                            continue;
                        }
                        int result2=result-num[j];
                        int left=j+1,right=num.length-1;
                        while(left<right){
                            int sum=num[left]+num[right];
                            if(sum<result2){
                                left++;
                            }else if(sum>result2){
                                right--;
                            }else{
                                ArrayList<Integer> list=new ArrayList<Integer>();
                                list.add(num[i]);
                                list.add(num[j]);
                                list.add(num[left]);
                                list.add(num[right]);
                                if(!all.contains(list)){
                                     all.add(list);
                                }
                                right--;
                            }
                        }
                }
            }
        return all;
    }
}
参考：https://www.nowcoder.com/profile/9319545/codeBookDetail?submissionId=24444004
        https://www.cnblogs.com/grandyang/p/4130379.html