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;//若前兩個數爲4，5，目標爲0，終止
           if(target<0&&target<2*(num[i]+num[j]))break;//若前兩個數爲-5，-4，目標小於-18，終止
           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;
    }
}