3Sum

 1 public class Solution {
 2     public ArrayList<ArrayList<Integer>> threeSum(int[] num) {
 3         if(num == null)
 4             return null;
 5         
 6         Arrays.sort(num);
 7         HashSet<ArrayList<Integer>> set = new HashSet<ArrayList<Integer>>();
 8         
 9         for(int i = 0; i < num.length; i++){
10             int start = i+1;
11             int end = num.length-1;
12             
13             while(start < end){
14                 int sum = num[i] + num[start] + num[end];
15                 if(sum == 0){
16                     ArrayList<Integer> tmp = new ArrayList<Integer>();
17                     tmp.add(num[i]);
18                     tmp.add(num[start]);
19                     tmp.add(num[end]);
20                     set.add(tmp);
21                     start++;
22                     end--;
23                 }
24                 else if(sum < 0)
25                     start++;
26                 else
27                     end--;
28             }
29         }
30         
31         ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
32         result.addAll(set);
33         return result;
34         
35     }
36 }