🔑 Longest Substring Without Repeating Characters – Java Cheat Sheet (One Page)

📌 What Is It?

The problem involves finding the length of the longest substring in a string `s` that contains no repeating characters.

Uses the Sliding Window technique
Maintains a set or map to track characters in the current window
Expands and shrinks the window dynamically

🧱 Pattern Template

public int lengthOfLongestSubstring(String s) {
        Map lastSeen = new HashMap<>();
        int maxLen = 0;
        int left = 0;

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

            if (lastSeen.containsKey(c) && lastSeen.get(c) >= left) {
                left = lastSeen.get(c) + 1; // jump left forward
            }

            lastSeen.put(c, right);
            maxLen = Math.max(maxLen, right - left + 1);
        }

        return maxLen;
}

✅ Use Cases

Finding substrings with unique characters
Text processing problems
Dynamic window size problems

📘 Common LeetCode Problems

3: Longest Substring Without Repeating Characters [Medium] Sliding Window, HashMap
340: Longest Substring with At Most K Distinct Characters [Medium] Sliding Window, HashMap

🧪 Example: Longest Substring Without Repeating Characters

Input: s = "abcabcbb"
Output: 3
Explanation: The answer is "abc", with the length of 3.

💡 Pro Tips

Use a frequency map or set to track characters in the current window
Expand the window to include characters, shrink to remove duplicates
Edge cases: empty string, all unique characters