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