15. 三数之和

例如, 给定数组 nums = [-1, 0, 1, 2, -1, -4]，

满足要求的三元组集合为：
[
  [-1, 0, 1],
  [-1, -1, 2]
]

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