TrappingRainwater.java

//https://leetcode.com/problems/trapping-rain-water/

class Solution {
    //T(n): O(n)
    //S(n): O(n)
    public int trap(int[] height) {
        int n = height.length;
        int[] left = new int[n];
        int[] right = new int[n];
        
        left[0] = height[0];
        for (int i = 1; i < n; i++) {
            if (height[i] > left[i - 1]) {
                left[i] = height[i];
            } else {
                left[i] = left[i - 1];
            }
        }
        
        right[n - 1] = height[n - 1];
        for (int i = n - 2; i >= 0; i--) {
            if (height[i] > right[i + 1]) {
                right[i] = height[i];
            } else {
                right[i] = right[i + 1];
            }
        }
        
        int sum = 0;
        for (int i = 0; i < n; i++) {
            height[i] = Math.min(left[i], right[i]) - height[i];
            sum += height[i];
        }
        return sum;
    }
}