Merge Two Sorted Lists
<p>You are given the heads of two sorted linked lists <code>list1</code> and <code>list2</code>.</p>

<p>Merge the two lists into one <strong>sorted</strong> list. The list should be made by splicing together the nodes of the first two lists.</p>

<p>Return <em>the head of the merged linked list</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0000-0099/0021.Merge%20Two%20Sorted%20Lists/images/merge_ex1.jpg" style="width: 662px; height: 302px;" />
<pre>
<strong>Input:</strong> list1 = [1,2,4], list2 = [1,3,4]
<strong>Output:</strong> [1,1,2,3,4,4]
</pre>

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

<pre>
<strong>Input:</strong> list1 = [], list2 = []
<strong>Output:</strong> []
</pre>

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

<pre>
<strong>Input:</strong> list1 = [], list2 = [0]
<strong>Output:</strong> [0]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in both lists is in the range <code>[0, 50]</code>.</li>
	<li><code>-100 &lt;= Node.val &lt;= 100</code></li>
	<li>Both <code>list1</code> and <code>list2</code> are sorted in <strong>non-decreasing</strong> order.</li>
</ul>
Recursion; Linked List
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
        if (!list1) return list2;
        if (!list2) return list1;
        if (list1->val <= list2->val) {
            list1->next = mergeTwoLists(list1->next, list2);
            return list1;
        } else {
            list2->next = mergeTwoLists(list1, list2->next);
            return list2;
        }
    }
};