42. 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 * 104
• 0 <= height[i] <= 105

---

class Solution:
    
    def trap(self, height: List[int]) -> int:
        lmax = [0] * len(height)
        rmax = [0] * len(height)
        clmax = 0
        crmax=0
        for i in range(len(height)-1):
            lmax[i+1] = clmax = max(clmax,height[i])
            rmax[i+1] = crmax = max(crmax,height[-1-i])
        rmax = rmax[::-1]
        for i in range(len(height)):
            height[i] = max(min(lmax[i],rmax[i])-height[i], 0 )
        return sum(height)