Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note:

• Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ≤ b ≤ c)
• The solution set must not contain duplicate triplets.

    For example, given array S = {-1 0 1 2 -1 -4},

    A solution set is:
    (-1, 0, 1)
    (-1, -1, 2)

题目：给定一个数组S，其中的元素a,b,c是否使得a+b+c=0成立？找出所有的和为0的唯一组合。

	public static List<List<Integer>> threeSum(int[] num) {
		List<List<Integer>> list = new ArrayList<List<Integer>>();
		int len = num.length;
		if (len < 3)
			return list;
		Arrays.sort(num);
		Set set = new HashSet();
		int j = 0, k = 0;
		for (int i = 0; i < len - 2; i++) {
			int target = -num[i];
			j = i + 1;
			k = len - 1;
			while (j < k) {
				if (num[j] + num[k] == target) {
					List<Integer> li = new ArrayList<Integer>();
					li.add(num[i]);
					li.add(num[j]);
					li.add(num[k]);
					if (set.add(li))
						list.add(li);
					j++;
					k--;
				} else if (num[j] + num[k] < target)
					j++;
				else
					k--;
			}
		}
		return list;
	}

LeetCode——3Sum