42. Trapping Rain Water

Problem Statement

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:

Rust

impl Solution {
    pub fn trap(height: Vec<i32>) -> i32 {
        let (mut left, mut right) = (0, height.len() - 1);
        let (mut left_max, mut right_max) = (0, 0);
        let mut ans = 0;

        // iterate from both sides to the middle
        while left < right {
            // if left is lower than right, then the water level depends on left
            // else the water level depends on right
            if height[left] < height[right] {
                // if left height is higher than left_max, then update left_max
                // else add the difference between left_max and left height to ans
                if height[left] >= left_max {
                    left_max = height[left];
                } else {
                    ans += left_max - height[left];
                }
                left += 1;
            } else {
                // if right height is higher than right_max, then update right_max
                // else add the difference between right_max and right height to ans
                if height[right] >= right_max {
                    right_max = height[right];
                } else {
                    ans += right_max - height[right];
                }
                right -= 1;
            }
        }

        ans
    }
}

42. Trapping Rain Water #

Problem Statement #

Solution: #

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42. Trapping Rain Water

Problem Statement

Solution:

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