CODE 114. 4Sum

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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)

public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
	// IMPORTANT: Please reset any member data you declared, as
	// the same Solution instance will be reused for each test case.
	if (num.length <= 3) {
		ArrayList<ArrayList<Integer>> results = new ArrayList<ArrayList<Integer>>();
		return results;
	}
	ArrayList<ArrayList<Integer>> results = new ArrayList<ArrayList<Integer>>();
	Arrays.sort(num);
	for (int i = 0; i < num.length - 3; i++) {
		for (int j = i + 3; j < num.length; j++) {
			int sum = num[i] + num[j];
			int start = i + 1;
			int end = j - 1;
			while (start < end) {
				if (num[start] + num[end] > target - sum) {
					while (start < end && num[end - 1] == num[end]) {
						end--;
					}
					end--;
				} else if (num[start] + num[end] < target - sum) {
					while (start < end && num[start + 1] == num[start]) {
						start++;
					}
					start++;
				} else {
					ArrayList<Integer> result = new ArrayList<Integer>();
					result.add(num[i]);
					result.add(num[start]);
					result.add(num[end]);
					result.add(num[j]);
					if (!results.contains(result)) {
						results.add(result);
					}
					start++;
					end--;
				}
			}
		}

		while (i < num.length - 3 && num[i + 1] == num[i]) {
			i++;
		}
	}
	return results;
}