21. Merge Two Sorted Lists - leetcode - python - easy

 You are given the heads of two sorted linked lists list1 and list2.

Merge the two lists into one sorted list. The list should be made by splicing together the nodes of the first two lists.

Return the head of the merged linked list.

 

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.

class Solution:

    def mergeTwoLists(self, l1: Optional[ListNode],

l2: Optional[ListNode]) -> Optional[ListNode]:

        dummy = cur = ListNode()

        while l1 and l2:

            if l1.val < l2.val:

                cur.next, l1 = l1, l1.next

            else:

                cur.next, l2 = l2, l2.next

            cur = cur.next

        cur.next = l1 or l2

        return dummy.next

Explanation (Simple Version) Create a dummy node to simplify edge cases. Use cur to build the merged list. Compare values of l1 and l2: Append the smaller one to cur and move that list forward. After the loop, attach the remaining list (l1 or l2). Return dummy.next → the merged list head. ✅ Compact, clean, and easy to remember.