[LeetCode] 3Sum

Thought:
Sort and iterate the array, for every element, we find the other two elements whose sum is -num[i], it is a O(n^2) solution.

Code:
public class Solution {
    public ArrayList<ArrayList<Integer>> threeSum(int[] num) {
        HashSet<ArrayList<Integer>> result = new HashSet<ArrayList<Integer>>();
        Arrays.sort(num);
        for(int i = 0;i < num.length; i++){
            int j = i + 1;
            int k = num.length - 1;
            while(j < k){
                if(num[j] + num[k] + num[i] < 0){
                    j++;
                }else if(num[j] + num[k] + num[i] > 0){
                    k--;
                }else{
                    ArrayList<Integer> tmp = new ArrayList<Integer>();                  
                    tmp.add(num[i]);
                    tmp.add(num[j]);
                    tmp.add(num[k]);
                    result.add(tmp);
                    j++;
                    k--;
                }               
            }
        }
        return new ArrayList<ArrayList<Integer>>(result);
    }
}

Code when I met this problem the second time:

public class Solution {
    public ArrayList<ArrayList<Integer>> threeSum(int[] num) {
        Set<ArrayList<Integer>> result = new HashSet<ArrayList<Integer>>();
        Arrays.sort(num);
        for (int i = 0; i < num.length; i++) {
            twoSum(Arrays.copyOfRange(num, i + 1, num.length), -num[i], result);
        }
        return new ArrayList<ArrayList<Integer>>(result);
    }
    public void twoSum(int[] array, int sum, Set<ArrayList<Integer>> result) {
        if (array.length < 2) return;
        int i = 0, j = array.length - 1;
        while (i < j) {
            if (array[i] + array[j] < sum) i++;
            else if (array[i] + array[j] > sum) j--;
            else {
                ArrayList<Integer> temp = new ArrayList<Integer>();
                temp.add(-sum);
                temp.add(array[i]);
                temp.add(array[j]);
                result.add(temp);
                i++;       // I forgot this two lines at first...............
                j--;
            }
        }
    }
}