21-MergeTwoSortedLists

Merge Two Sorted Lists

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.

Problem can be found in here!

Solutions

# Definition for singly-linked list.
class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

Solution: Dummy Head

def mergeTwoLists(list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
    dummy_head = current_node = ListNode()

    while list1 and list2:
        if list1.val < list2.val:
            current_node.next = list1
            list1 = list1.next
        else:
            current_node.next = list2
            list2 = list2.next
        current_node = current_node.next

    current_node.next = list1 if list1 else list2
    return dummy_head.next

Time Complexity: $O(n+m)$, Space Complexity: $O(1)$, where n and m are the length of list1 and list2, respectively.