add leetcode problem-cn part6

算法题（国内版）/problem (English)/两两交换链表中的节点(English) [swap-nodes-in-pairs].html

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+<p>Given a&nbsp;linked list, swap every two adjacent nodes and return its head. You must solve the problem without&nbsp;modifying the values in the list&#39;s nodes (i.e., only nodes themselves may be changed.)</p>
+
+<p>&nbsp;</p>
+<p><strong>Example 1:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2020/10/03/swap_ex1.jpg" style="width: 422px; height: 222px;" />
+<pre>
+<strong>Input:</strong> head = [1,2,3,4]
+<strong>Output:</strong> [2,1,4,3]
+</pre>
+
+<p><strong>Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> head = []
+<strong>Output:</strong> []
+</pre>
+
+<p><strong>Example 3:</strong></p>
+
+<pre>
+<strong>Input:</strong> head = [1]
+<strong>Output:</strong> [1]
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li>The number of nodes in the&nbsp;list&nbsp;is in the range <code>[0, 100]</code>.</li>
+	<li><code>0 &lt;= Node.val &lt;= 100</code></li>
+</ul>