3. Longest Substring Without Repeating Characters

Keep a window [l, r] over the string such that it never contains a duplicate. When you find s[r] was seen inside the current window, jump l just past the previous occurrence — that's the smallest move that restores the invariant.

Approach

1. last = {} maps char to its latest index.
2. Initialize l = 0, best = 0.
3. For each r from 0 to n-1:
   • If s[r] in last and last[s[r]] >= l: set l = last[s[r]] + 1.
   • Update last[s[r]] = r.
   • best = max(best, r - l + 1).

Walk-through

s = "abcabcbb"

import java.util.*;

class Solution {
    public int lengthOfLongestSubstring(String s) {
        Map<Character, Integer> last = new HashMap<>();
        int l = 0, best = 0;
        for (int r = 0; r < s.length(); r++) {
            char ch = s.charAt(r);
            if (last.containsKey(ch) && last.get(ch) >= l) {
                l = last.get(ch) + 1;
            }
            last.put(ch, r);
            best = Math.max(best, r - l + 1);
        }
        return best;
    }
}