21. Merge Two Sorted Lists {Easy

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

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

class Solution:
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        
        root = ListNode(0)
        head = root
        while l1 is not None or l2 is not None:
            if l1 is None:
                head.next = l2
                return root.next
            elif l2 is None:
                head.next = l1
                return root.next
            else:
                if l1.val <= l2.val:
                    head.next = ListNode(l1.val)
                    l1 = l1.next
                else:
                    head.next = ListNode(l2.val)
                    l2 = l2.next
                head = head.next
        return root.next