Today I have done leetcode's September LeetCoding Challenge with cpp.

September LeetCoding Challenge 18

Description

Trapping Rain Water

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it can trap after raining.

Example 1:

Input: height = [0,1,0,2,1,0,1,3,2,1,2,1]
Output: 6
Explanation: The above elevation map (black section) is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped.

Example 2:

Input: height = [4,2,0,3,2,5]
Output: 9

Constraints:

• n == height.length
• 1 <= n <= 2 * 10^4
• 0 <= height[i] <= 10^5

Solution

auto speedup = [](){
  cin.tie(nullptr);
  cout.tie(nullptr);
  ios::sync_with_stdio(false);
  return 0;
}();
class Solution {
public:
  int trap(vector<int>& height) {
    vector<int> maxRights{0};
    int len = height.size();
    for(int i = len - 1; i >= 0; --i) {
      if(height[i] >= maxRights.back()) {
        maxRights.push_back(height[i]);
      }
    }
    int maxLeft = 0;
    int answer = 0;
    for(auto h : height) {
      maxLeft = max(maxLeft, h);
      int maxRight = maxRights.back();
      int waterHeight = maxLeft > maxRight ? maxRight : maxLeft;
      answer += waterHeight > h ? (waterHeight - h) : 0;
      if(h == maxRight) {
        maxRights.pop_back();
      }
    }
    return answer;
  }
};

// Accepted
// 322/322 cases passed (21 ms)
// Your runtime beats 72.47 % of cpp submissions
// Your memory usage beats 43.13 % of cpp submissions (20.2 MB)