18. 4Sum

public class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
 List<List<Integer>> rst = new ArrayList<List<Integer>>();
		Arrays.sort(nums);

		for (int i = 0; i < nums.length - 3; i++) {
			if (i != 0 && nums[i] == nums[i - 1]) {
				continue;
			}

			for (int j = i + 1; j < nums.length - 2; j++) {
				if (j != i + 1 && nums[j] == nums[j - 1])
					continue;

				int left = j + 1;
				int right = nums.length - 1;
				while (left < right) {
					int sum = nums[i] + nums[j] + nums[left] + nums[right];
					if (sum < target) {
						left++;
					} else if (sum > target) {
						right--;
					} else {
						ArrayList<Integer> tmp = new ArrayList<Integer>();
						tmp.add(nums[i]);
						tmp.add(nums[j]);
						tmp.add(nums[left]);
						tmp.add(nums[right]);
						rst.add(tmp);
						left++;
						right--;
						while (left < right && nums[left] == nums[left - 1]) {
							left++;
						}
						while (left < right && nums[right] == nums[right + 1]) {
							right--;
						}
					}
				}
			}
		}

		return rst;
	
    }
}