Trapping Rain Water - Leetcode 42

Trapping Rain Water - Leetcode 42


3 min read

Problem - Leetcode

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 rain.

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


  • n == height.length

  • 1 <= n <= 2 * 10<sup>4</sup>

  • 0 <= height[i] <= 10<sup>5</sup>

Answer-1 in Golang

func trap(height []int) int {
    if height == nil {
        return 0
    left, right := 0, len(height)-1
    leftMax, rightMax := height[left], height[right]
    result := 0

    for left < right {
        if leftMax < rightMax {
            leftMax = max(leftMax, height[left])
            result = result + leftMax - height[left]
        } else {
            rightMax = max(rightMax, height[right])
            result = result + rightMax - height[right]
    return result

func max(a int, b int) int {
    if a > b {
        return a
    return b

This code defines a function trap in Go that calculates the amount of trapped rainwater between bars of varying heights represented by the height array. It uses a two-pointer approach to find the trapped water. Here's a detailed breakdown of the code:

  1. The trap function takes an input array height which represents the heights of bars.

  2. It checks if the input array is nil, and if so, returns 0 (since there's no trapped water if there are no bars).

  3. Initialize two pointers left and right to the beginning and end of the height array, respectively.

  4. Initialize two variables leftMax and rightMax to store the maximum height encountered on the left and right sides, both initialized to the heights of the first and last bars.

  5. Initialize a variable result to store the total trapped rainwater, initially set to 0.

  6. Enter a for loop while the left pointer is less than the right pointer:

    1. Compare leftMax and rightMax to determine which side has a smaller maximum height.

    2. If leftMax is smaller, increment the left pointer, update leftMax to the new maximum height, and add the difference between leftMax and the height at height[left] to result.

    3. If rightMax is smaller (or equal), decrement the right pointer, update rightMax to the new maximum height, and add the difference between rightMax and the height at height[right] to result.

  7. Once the for loop ends (when left is no longer less than right), return the result variable, which contains the total trapped rainwater.

  8. The max function is a utility function that takes two integers a and b and returns the maximum of the two.

The overall idea behind this code is to calculate the trapped rainwater by comparing the heights of the bars on the left and right sides. It iterates through the bars using the two-pointer approach and keeps track of the maximum heights encountered. By calculating the difference between the maximum height and the current height at each step, it accurately calculates the trapped rainwater.

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