Given a string `s`, find the length of the **longest substring** without repeating characters.
**Example 1:**
```
Input: s = "abcabcbb"
Output: 3
Explanation: The answer is "abc", with length 3.
```
**Example 2:**
```
Input: s = "bbbbb"
Output: 1
```
**Example 3:**
```
Input: s = "pwwkew"
Output: 3
```