leetcode : 4 sum (再次提交，超时)

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: 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]
]

tag : two points

public class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
		if(nums == null || nums.length < 4){
			return result;
		}

		Arrays.sort(nums);

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

			int j;

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

				int left = j + 1;
				int right = nums.length - 1;

				while(left < right){
					int sum = a + b + nums[left] + nums[right];
					if(sum == target){
						List<Integer> list = new ArrayList<Integer>();
						list.add(a);
						list.add(b);
						list.add(nums[left]);
						list.add(nums[right]);
						result.add(list);
						left++;
						right--;
						while(left < right && nums[left] == nums[left-1]){
							left++;
						}
						while(left < right && nums[right+1] == nums[right]){
							right--;
						}
					}else if(sum < target){
						left++;
					}else{
						right--;
					}
				}
			}

		}

		return result;

    }
}