Given a string, find the length of the **longest substring** without repeating characters.

**Examples:**

Given `"abcabcbb"`

, the answer is `"abc"`

, which the length is 3.

Given `"bbbbb"`

, the answer is `"b"`

, with the length of 1.

Given `"pwwkew"`

, the answer is `"wke"`

, with the length of 3. Note that the answer must be a **substring**, `"pwke"`

is a *subsequence *and not a substring.

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