Given a string, find the length of the longest substring without repeating characters.
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Given "abcabcbb", the answer is "abc", which the length is 3.
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Given "bbbbb", the answer is "b", with the length of 1.
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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.