Longest Substring with At Most Two Distinct Characters

/*
 * Problem : Longest part with two characters.
 * Input   : One word.
 * Output  : The length.
 * Approach: Sliding window.
 * 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 int solve(String s) {
        // Step 1: The window is s[left..right]. "counts" says how many times each character is inside the window.
        java.util.Map<Character, Integer> counts = new java.util.HashMap<Character, Integer>();
        int left = 0;
        int best = 0;
        for (int right = 0; right < s.length(); right++) {
            counts.merge(s.charAt(right), 1, Integer::sum);
            // Step 2: Too many different characters: move the left side until only two are left.
            while (counts.size() > 2) {
                char out = s.charAt(left++);
                if (counts.merge(out, -1, Integer::sum) == 0) {
                    counts.remove(out);
                }
            }
            best = Math.max(best, right - left + 1);
        }
        return best;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String s = scanner.hasNextLine() ? scanner.nextLine() : "";
        System.out.println(solve(s));
    }
}