<h2><a href="https://leetcode.com/problems/swap-nodes-in-pairs/">24. Swap Nodes in Pairs</a></h2><h3>Medium</h3><hr><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 class="example">Example 1:</strong></p>

<div class="example-block">

<p><strong>Input:</strong> <span class="example-io">head = [1,2,3,4]</span></p>

<p><strong>Output:</strong> <span class="example-io">[2,1,4,3]</span></p>

<p><strong>Explanation:</strong></p>

<p><img alt="" src="https://assets.leetcode.com/uploads/2020/10/03/swap_ex1.jpg" style="width: 422px; height: 222px;" /></p>

</div>

<p><strong class="example">Example 2:</strong></p>

<div class="example-block">

<p><strong>Input:</strong> <span class="example-io">head = []</span></p>

<p><strong>Output:</strong> <span class="example-io">[]</span></p>

</div>

<p><strong class="example">Example 3:</strong></p>

<div class="example-block">

<p><strong>Input:</strong> <span class="example-io">head = [1]</span></p>

<p><strong>Output:</strong> <span class="example-io">[1]</span></p>

</div>

<p><strong class="example">Example 4:</strong></p>

<div class="example-block">

<p><strong>Input:</strong> <span class="example-io">head = [1,2,3]</span></p>

<p><strong>Output:</strong> <span class="example-io">[2,1,3]</span></p>

</div>

<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>