# 3. Longest Substring Without Repeating Characters

## Intitution

The problem asks for the length of the longest substring with no repeating characters. We can approach this by using the **sliding window technique**. As we traverse the string, we maintain a window of characters with no duplicates. When we encounter a duplicate, we shrink the window from the left until the duplicate is removed. Throughout, we keep track of the maximum window size observed.



## Complexity

| Space Complexity | Time Complexity |
| ---------------- | --------------- |
| $$\text{O}(N)$$  | $$\text{O}(N)$$ |



## Code

```java
public int lengthOfLongestSubstring(String input) {
    // Array to keep track of seen characters in the current window
    boolean[] isCharSeen = new boolean[256];

    int maxSubstringLength = 0;

    // Sliding window boundaries
    for (int windowStart = 0, windowEnd = 0; windowEnd < input.length(); ++windowEnd) {
        // If character at windowEnd has already been seen, shrink the window from the start
        while (isCharSeen[input.charAt(windowEnd) - '\0'] && windowStart < windowEnd) {
            isCharSeen[input.charAt(windowStart++) - '\0'] = false;
        }

        // Update the maximum length if this window is longer
        maxSubstringLength = Math.max(windowEnd - windowStart + 1, maxSubstringLength);

        // Mark the current character as seen
        isCharSeen[input.charAt(windowEnd) - '\0'] = true;
    }

    return maxSubstringLength;
}

```
