3Sum

public class Solution {
    public ArrayList<ArrayList<Integer>> threeSum(int[] num) {
        if(num == null)
            return null;
        
        Arrays.sort(num);
        HashSet<ArrayList<Integer>> set = new HashSet<ArrayList<Integer>>();
        
        for(int i = 0; i < num.length; i++){
            int start = i+1;
            int end = num.length-1;
            
            while(start < end){
                int sum = num[i] + num[start] + num[end];
                if(sum == 0){
                    ArrayList<Integer> tmp = new ArrayList<Integer>();
                    tmp.add(num[i]);
                    tmp.add(num[start]);
                    tmp.add(num[end]);
                    set.add(tmp);
                    start++;
                    end--;
                }
                else if(sum < 0)
                    start++;
                else
                    end--;
            }
        }
        
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        result.addAll(set);
        return result;
        
    }
}