Trapping Rain Water
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 elevation map is represented by array [0,1,0,2,1,0,1,3,2,1,2,1].
In this case, 6 units of rain water are being trapped.
Example 2:
Input: height = [4,2,0,3,2,5]
Output: 9
Explanation: 9 units of rain water are trapped between the bars.
Examples
Example 1
Input: height = [0,1,0,2,1,0,1,3,2,1,2,1]
Output: 6
Explanation: Water fills the valleys between bars. The total trapped water is 6 units.
Example 2
Input: height = [4,2,0,3,2,5]
Output: 9
Explanation: Water fills the gaps between the tall bars on each side, trapping 9 units total.
Example 3
Input: height = [1,0,1]
Output: 1
Explanation: One unit of water is trapped between the two bars of height 1.
Constraints
- -n == height.length
- -1 <= n <= 2 * 10^4
- -0 <= height[i] <= 10^5
Optimal Complexity
Time
O(n)
Space
O(1)
Practice this problem with an AI interviewer
TechInView conducts a full voice mock interview — the AI asks clarifying questions, evaluates your approach, watches you code, and scores you on 5 dimensions. Just like a real FAANG interview.
Start a free interview