LeetCode 42. Trapping Rain Water

Description

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.

For example,
Given [0,1,0,2,1,0,1,3,2,1,2,1], return 6.

Explanation

The key point is to find the relationship between bar heights and amount of the water.

We can find the relationship by looking at each individual bar and get the amount of water it can hold. Then sum up all water bar hold together.

Video Tutorial

Java Solution

class Solution {
    public int trap(int[] height) {
        int totalAmount = 0;
        if (height == null || height.length == 0) {
            return totalAmount;
        }
        
        int[] leftHighest = new int[height.length + 1];
        leftHighest[0] = 0;        
        for (int i = 0; i < height.length; i++) {
            leftHighest[i + 1] = Math.max(leftHighest[i], height[i]);            
        }
        
        int rightHighest = 0;
        for (int i = height.length - 1; i >= 0; i--) {
            rightHighest = Math.max(height[i], rightHighest);
            totalAmount += Math.min(leftHighest[i], rightHighest) > height[i] ? Math.min(leftHighest[i], rightHighest) - height[i] : 0;            
        }
        
        return totalAmount;
    }
}