4Sum

public class Solution {    public ArrayList<ArrayList<Integer>> fourSum(int[] num,int target) {        // Start typing your Java solution below        // DO NOT write main() function       if(num==null||num.length<4)return new ArrayList<ArrayList<Integer>>();       int len = num.length;       Arrays.sort(num);       ArrayList<ArrayList<Integer>> ret = new ArrayList<ArrayList<Integer>>();       for(int i = 0; i < len-3;i++)          for(int j=i+1;j<len-2;j++){           if(target>=0&&num[i]+num[j]>target) break;//若前两个数为4，5，目标为0，终止           if(target<0&&target<2*(num[i]+num[j]))break;//若前两个数为-5，-4，目标小于-18，终止           int mid = j+1;           int right = len-1;           int sum = target-num[i]-num[j];           while(mid<right){               if(num[mid]+num[right]>sum)right--;               else if(num[mid]+num[right]==sum){                   ArrayList cur = new ArrayList();                   cur.add(num[i]);cur.add(num[j]);cur.add(num[mid]);cur.add(num[right]);                   if(!ret.contains(cur))                   ret.add(cur);                   //while(right>0&&num[right-1]==num[right])right--;                   //while(mid<len-1&&num[mid+1]==num[mid])mid++;                   mid++;right--;               }               else mid++;           }           //while(j<len-2&&num[j+1]==num[j])j++;       }       return ret;    }}