# LeetCode 3: Longest Substring Without Repeating Characters — Step-by-Step Visual Trace

**Medium** — Sliding Window | Two Pointers | Hash Set | String

## The Problem

Find the length of the longest substring within a given string that contains all unique characters with no repeating characters.

## Approach

Use a sliding window technique with two pointers (left and right) and a set to track unique characters. Expand the window by moving the right pointer when characters are unique, and contract by moving the left pointer when duplicates are found.

**Time:** O(n) · **Space:** O(min(m, n))

## Code

```python
class Solution:
    def lengthOfLongestSubstring(self, s: str) -> int:
        left, right = 0, 0
        max_length = 0
        unique_chars = set()

        while right < len(s):
            if s[right] not in unique_chars:
                unique_chars.add(s[right])
                max_length = max(max_length, right - left + 1)
                right += 1
            else:
                unique_chars.remove(s[left])
                left += 1

        return max_length
```

## Watch It Run

> **[Open interactive visualization](https://tracelit.dev/app?trace=0003_longest-substring-without-repeating-characters)**

> **Try it yourself:** Open [TraceLit](https://tracelit.dev/app?trace=0003_longest-substring-without-repeating-characters) and step through every line.

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*Built with [TraceLit](https://tracelit.dev) — the visual algorithm tracer for LeetCode practice.*