Program to find how many total amount of rain we can catch in Python

Suppose we have an array of n non-negative integers. These are representing a height where the width of each bar is 1, we have to compute how much water it is able to catch after raining. So the map will be like −

Here we can see there are 8 blue boxes, so the output will be 8.

To solve this, we will follow these steps −

• Define a stack st, water := 0 and i := 0
• while i < size of height
  • if is stack is empty or height[stack top] >= height[i], then push i into stack, increase i by 1
  • otherwise
    • x := stack top element, delete top from stack
    • if stack is not empty, then
      • temp := min of height[stack top element] and height[i]
      • dest := i – stack top element – 1
      • water := water + dist * (temp – height[x])
• return water

Let us see the following implementation to get better understanding −

Example

class Solution(object):
   def trap(self, height):
      stack = []
      water = 0
      i=0
      while i<len(height):
         if len(stack) == 0 or height[stack[-1]]>=height[i]:
            stack.append(i)
            i+=1
         else:
            x = stack[-1]
            stack.pop()
            if len(stack) != 0:
               temp = min(height[stack[-1]],height[i])
               dist = i - stack[-1]-1
               water += dist*(temp - height[x])
      return water
ob = Solution()
print(ob.trap([2,5,2,0,5,8,8]))

Input

[2,5,2,0,5,8,8]

Output

8