message: Update trapping-rain-water.cpp
diff: // Space: O(1)
  class Solution {
  +public:
  + int trap(vector<int>& height) {
  + int result = 0, left = 0, right = height.size()-1, level = 0;
  + while (left < right) {
  + int lower = height[height[left] < height[right] ? left++ : right--];
  + level = max(level, lower);
  + result += level - lower;
  + }
  + return result;
  + }
  +};
  +
  +// Time: O(n)
  +// Space: O(1)
  +class Solution2 {
  public:
  int trap(vector<int>& height) {
  if (height.empty()) {
  