15. 三数之和(双指针)

public List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> ans = new ArrayList<>();
        Arrays.sort(nums);
        if (nums.length < 2) return ans;
        if (nums[0] > 0) return ans;
        for (int i = 0; i < nums.length - 2; i++) {
            if (i > 0 && nums[i] == nums[i - 1]) continue;
            int target = -nums[i];
            int left = i + 1, right = nums.length - 1;
            while (left < right) {
                if (nums[left] + nums[right] < target) left++;
                else if (nums[left] + nums[right] == target) {
                    ans.add(new ArrayList<>(Arrays.asList(nums[i], nums[left], nums[right])));
                    left++;
                    right--;
                    while (left < right && nums[left] == nums[left - 1]) left++;
                    while (left < right && nums[right] == nums[right + 1]) right--;
                } else right--;
            }
        }
        return ans;
    }