[Python][LeetCode] 42. Trapping Rain Water

Link

class Solution:
    def trap(self, height: List[int]) -> int:
        l = 0
        r = len(height) - 1
        
        # Those are for initializing 
        left_max_height = 0
        right_max_height = 0
        total_water = 0

        while l < r: 
            if height[l] < height[r]: # Point 1. This is one of the 'any' condition
                                      # just for checkiing all the elements for the list.
                if left_max_height <= height[l]:
                    left_max_height = height[l]
                else:
                    total_water += (left_max_height - height[l])
                l += 1  
            else:  ###### marking 1 (point 1's)
              
              # Point 2. Basically, if the next index's height is 'higher' than before one,
              # we just have to renew the variable of varialbe of 'right_max_height' and moving the index of 'r'.
                if right_max_height <= height[r]:
                    right_max_height = height[r]
                    
            # Point3. Basically, if the next index's height is lower than before one,
            # we have to add 'that difference' to variable of 'total_water'.
                else: 
                    total_water += (right_max_height - height[r]) 
                r -= 1 ###### marking 2 (point 1's)
        return total_water

Summary

1. while l < r: 
       if height[l] < height[r]: 
   

   This is one of the ‘any’ condition just for checkiing all the elements for the list by moving index of ‘l’ and ‘r’. (Pay attention to the indentation! I marked them!)

2.                 if right_max_height <= height[r]:
                       right_max_height = height[r]
   

   Basically, if the next index’s height is ‘higher’ than before one, we just have to renew the variable of varialbe of ‘right_max_height’ and moving the index of ‘r’. (The case of ‘left’ is almost same with this.)

3.                 else: 
                       total_water += (right_max_height - height[r]) 
                   r -= 1 
   

Basically, if the next index’s height is lower than before one, we have to add ‘that difference’ to variable of ‘total_water’.(The case of ‘left’ is almost same with this.)