Longest Substring Without Repeating Characters

import java.util.*;

public class LongestSubstring {
    public static void main(String[] args) {
        System.out.println(lengthOfLongestSubstring("abcabcbb"));    // 3
        System.out.println(lengthOfLongestSubstring("bbbbb"));       // 1
        System.out.println(lengthOfLongestSubstring("pwwkew"));      // 3
        System.out.println(lengthOfLongestSubstring("abba"));        // 2
        System.out.println(lengthOfLongestSubstring(""));            // 0
    }

    // Sliding window, O(n) time. The window is s[start..end] and never contains a repeated character.
    static int lengthOfLongestSubstring(String s) {
        Map<Character, Integer> lastSeen = new HashMap<>();
        int best = 0, start = 0;
        for (int end = 0; end < s.length(); end++) {
            Integer previous = lastSeen.put(s.charAt(end), end);
            if (previous != null && previous >= start) {
                start = previous + 1;                        // jump just past the earlier copy
            }
            best = Math.max(best, end - start + 1);
        }
        return best;
    }
}