docs(linked-list): add Merge Two Sorted Lists documentation page

apps/docs/content/(linked-list)/21-merge-two-sorted-lists.mdx

@@ -0,0 +1,66 @@
+---
+title: '21. Merge Two Sorted Lists'
+description: You are given the heads of two sorted linked lists list1 and list2
+sidebar:
+  label: 'Merge Two Sorted Lists'
+  badge: 'Easy'
+---
+
+<Badge variant="accent">Linked List</Badge>
+
+### Example 1:
+- Input: `list1 = [1,2,4], list2 = [1,3,4]`
+- Output: `[1,1,2,3,4,4]`
+
+### Example 2:
+- Input: `list1 = [], list2 = []`
+- Output: `[]`
+
+### Example 3:
+- Input: `list1 = [], list2 = [0]`
+- Output: `[0]`
+
+### Constraints:
+
+- The number of nodes in both lists is in the range [0, 50].
+- `-100 <= Node.val <= 100`
+- Both list1 and list2 are sorted in non-decreasing order.
+
+## Solution
+
+```py
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def mergeTwoLists(
+        self, list1: Optional[ListNode], list2: Optional[ListNode]
+    ) -> Optional[ListNode]:
+
+        if not list1:
+            return list2
+        if not list2:
+            return list1
+
+        if list1.val < list2.val:
+            head = list1
+            list1 = list1.next
+        else:
+            head = list2
+            list2 = list2.next
+
+        current = head
+        while list1 and list2:
+            if list1.val < list2.val:
+                current.next = list1
+                list1 = list1.next
+            else:
+                current.next = list2
+                list2 = list2.next
+            current = current.next
+
+        current.next = list1 or list2
+        return head
+```