Trapping Rain Water

/*
 * Problem : Process an array.
 * Input   : n, then n integers.
 * Output  : The trapped water.
 * Approach: Two pointers.
 * Time    : O(n)
 * Space   : O(1)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
    // This method solves the task. It receives the input and returns the answer.
    static long solve(int[] a) {
        // Step 1: Two pointers from both ends. leftMax and rightMax = the tallest bar seen from each side.
        int left = 0;
        int right = a.length - 1;
        int leftMax = 0;
        int rightMax = 0;
        long water = 0;

        while (left < right) {
            // Step 2: Work on the lower side. Its water level is limited by its own side maximum.
            if (a[left] < a[right]) {
                leftMax = Math.max(leftMax, a[left]);
                water += leftMax - a[left];
                left++;
            } else {
                rightMax = Math.max(rightMax, a[right]);
                water += rightMax - a[right];
                right--;
            }
        }
        return water;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int size = scanner.nextInt();
        int[] a = new int[size];
        for (int i = 0; i < size; i++) {
            a[i] = scanner.nextInt();
        }
        System.out.println(solve(a));
    }
}