Trapping Rain Water Problem LeetCode Java Solution

// SOLVING THIS WITH AN AI ASSISTANT (2026)

If you are working through this problem with an AI coding assistant — Claude, ChatGPT, Cursor chat, Gemini, GitHub Copilot, Aider, or any agent — the goal isn’t to ask for the answer. It is to use the tool to understand the pattern. The prompt sequence I’d run:

Spec it back to me first. “In your own words, what is this problem actually testing? What’s the smallest example that fails the naive approach?”
Brute-force first, optimize after. “Write the simplest correct solution, even if it’s O(n²). Don’t optimize. Just make it correct, with comments explaining each step.”
Ask for the upgrade. “Now show me the optimal solution. What insight makes it possible? What pattern is this an instance of?”
Stress-test it. “Generate 10 edge cases — empty input, single element, duplicates, max size, sorted, reverse-sorted. Run my solution against each.”

The pattern matters more than the answer. If the agent just hands you optimized code, you’ve trained yourself to lose interviews.

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining.

Example:

Input: [0,1,0,2,1,0,1,3,2,1,2,1]
Output: 6

class Solution {
    public int trap(int[] height) {
        int arrLength = height.length;
        int total = 0;
        if(arrLength<=2) return total;
        int[] leftMaxArr = new int[arrLength];
        int[] rightMaxArr = new int[arrLength];
        
        leftMaxArr = leftMax(leftMaxArr, height);
        rightMaxArr = rightMax(rightMaxArr, height);
        
        for(int i=0; i<arrLength; i++){
            if(leftMaxArr[i]<=rightMaxArr[i] && leftMaxArr[i]>height[i]){
                total = total + (leftMaxArr[i]-height[i]);
            }else if(rightMaxArr[i]<leftMaxArr[i] && rightMaxArr[i]>height[i] ){
                total = total + (rightMaxArr[i]-height[i]);
            }
        }
        
        return total;
        
    }
    
    public int[] leftMax(int[] arr, int[] height){
        int max = height[0];
        for(int i =0;i<height.length;i++){
            if(max>height[i]){
                    arr[i] = max;
            }else{
                arr[i] = height[i];
                max = height[i];
            }
            
        }
        return arr;

    }
    
    public int[] rightMax(int[] arr, int[] height){
        int max = height[height.length-1];
        for(int i =height.length-1;i>=0;i--){
            if(max>height[i]){
                    arr[i] = max;
            }else{
                arr[i] = height[i];
                max = height[i];
            }  
        }
        return arr;
    }
}

Here is the video explanation

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