Trapping Rain Water

Problem

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

Solution

• Two Pointers
• Time: O(n)
• Space: O(1)

class Solution(object):
    def trap(self, height):
        """
        :type height: List[int]
        :rtype: int
        """
        if not height: return 0

        l, r = 0, len(height) - 1
        leftMax, rightMax = height[l], height[r]
        res = 0

        while l < r:
            if leftMax < rightMax:
                l += 1
                leftMax = max(leftMax, height[l])
                res += leftMax - height[l]
            else:
                r -= 1
                rightMax = max(rightMax, height[r])
                res += rightMax - height[r]

        return res

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