Minimum Window Substring (Length)

/*
 * Problem : Shortest window.
 * Input   : Words s and t.
 * Output  : The length.
 * Approach: Sliding window
 * Time    : O(n + m)
 * 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 int solve(String a, String b) {
        // Step 1: need[c] is how many letters c are still missing in the window.
        int[] need = new int[128];
        for (int i = 0; i < b.length(); i++) {
            need[b.charAt(i)]++;
        }
        int missing = b.length();
        int left = 0;
        int best = 0;
        for (int right = 0; right < a.length(); right++) {
            // Step 2: Add a letter on the right. It reduces "missing" only if the window still needed it.
            if (need[a.charAt(right)]-- > 0) {
                missing--;
            }
            // Step 3: The window is valid when nothing is missing. Shrink it from the left as far as possible.
            while (missing == 0) {
                int size = right - left + 1;
                if (best == 0 || size < best) {
                    best = size;
                }
                if (++need[a.charAt(left++)] > 0) {
                    missing++;
                }
            }
        }
        return best;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String a = scanner.next();
        String b = scanner.next();
        System.out.println(solve(a, b));
    }
}