Longest Substring Without Repeating Characters

/*
 * Problem : Process one line of text.
 * Input   : One line of text.
 * Output  : Length of the longest unique window.
 * Approach: Sliding window.
 * Time    : O(n)
 * Space   : O(k)
 * 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: lastSeen remembers the last index of every character. -1 means: not seen yet.
        int[] lastSeen = new int[256];
        java.util.Arrays.fill(lastSeen, -1);

        // Step 2: The window is s[left..right]. It never has a repeated character.
        int left = 0;
        int best = 0;

        for (int right = 0; right < s.length(); right++) {
            char c = s.charAt(right);

            // Step 3: If c is already inside the window, move left behind its last position.
            if (lastSeen[c] >= left) {
                left = lastSeen[c] + 1;
            }

            // Step 4: Save the position of c and update the best length.
            lastSeen[c] = right;
            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));
    }
}