LeetCode-in-All

42. Trapping Rain Water

Hard

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

impl Solution {
    pub fn trap(height: Vec<i32>) -> i32 {
        let (mut l, mut r) = (0, height.len() as i32 - 1);
        let mut res = 0;
        let mut lower_wall = 0;

        while l < r {
            let l_val = height[l as usize];
            let r_val = height[r as usize];

            if l_val < r_val {
                lower_wall = lower_wall.max(l_val);
                res += lower_wall - l_val;
                l += 1;
            } else {
                lower_wall = lower_wall.max(r_val);
                res += lower_wall - r_val;
                r -= 1;
            }
        }

        res
    }
}

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