42.trapping-rain-water

 

Question:

 https://leetcode.com/problems/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
• 0 <= n <= 2 * 104
• 0 <= height[i] <= 105

Answer:

class Solution(object):
    def trap(self, height):
        """
        :type height: List[int]
        :rtype: int
        """
        left=[]
        right=[]
        left_max=0
        right_max=0
        for x in height:
               left_max=max(x,left_max)
               left.append(left_max)
        height.reverse()
        for y in height:
               right_max=max(y,right_max)
               right.append(right_max)
        right.reverse()
        output = 0
        for i in range(len(height)):
            output += min(right[i],left[i]) - height[i]
        return output