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

Note:

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

    For example, given array S = {1 0 -1 0 -2 2}, and target = 0.

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

题目：给定一个数组S，其中是否存在a+b+c+d=目标值。找出所有的不重复的组合。

	public List<List<Integer>> fourSum(int[] num, int target) {
		List<List<Integer>> list = new ArrayList<List<Integer>>();
		int len = num.length;
		if (len < 4)
			return list;
		Arrays.sort(num);
		Set set = new HashSet();
		int j = 0, k = 0, m = 1;
		for (int i = 0; i < len - 3; i++) {
			for (m = i + 1; m < len - 2; m++) {
				int left = target - (num[i] + num[m]);
				j = m + 1;
				k = len - 1;
				while (j < k) {
					if (num[j] + num[k] == left) {
						List<Integer> li = new ArrayList<Integer>();
						li.add(num[i]);
						li.add(num[m]);
						li.add(num[j]);
						li.add(num[k]);
						if (set.add(li))
							list.add(li);
						j++;
						k--;
					} else if (num[j] + num[k] < left)
						j++;
					else
						k--;
				}
			}
		}
		return list;
	}

LeetCode——4Sum