Added README.md file for Longest Substring Without Repeating Characters

3-longest-substring-without-repeating-characters/README.md

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+<h2><a href="https://leetcode.com/problems/longest-substring-without-repeating-characters">Longest Substring Without Repeating Characters</a></h2> <img src='https://img.shields.io/badge/Difficulty-Medium-orange' alt='Difficulty: Medium' /><hr><p>Given a string <code>s</code>, find the length of the <strong>longest</strong> <span data-keyword="substring-nonempty"><strong>substring</strong></span> without repeating characters.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> s = &quot;abcabcbb&quot;
+<strong>Output:</strong> 3
+<strong>Explanation:</strong> The answer is &quot;abc&quot;, with the length of 3.
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> s = &quot;bbbbb&quot;
+<strong>Output:</strong> 1
+<strong>Explanation:</strong> The answer is &quot;b&quot;, with the length of 1.
+</pre>
+
+<p><strong class="example">Example 3:</strong></p>
+
+<pre>
+<strong>Input:</strong> s = &quot;pwwkew&quot;
+<strong>Output:</strong> 3
+<strong>Explanation:</strong> The answer is &quot;wke&quot;, with the length of 3.
+Notice that the answer must be a substring, &quot;pwke&quot; is a subsequence and not a substring.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>0 &lt;= s.length &lt;= 5 * 10<sup>4</sup></code></li>
+	<li><code>s</code> consists of English letters, digits, symbols and spaces.</li>
+</ul>