21. Merge Two Sorted Lists

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

 

Example 1:

Input: l1 = [1,2,4], l2 = [1,3,4] Output: [1,1,2,3,4,4]

Example 2:

Input: l1 = [], l2 = [] Output: []

Example 3:

Input: l1 = [], l2 = [0] Output: [0]

 

Constraints:

• The number of nodes in both lists is in the range [0, 50].
• -100 <= Node.val <= 100
• Both l1 and l2 are sorted in non-decreasing order.

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution(object):
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        
        flag = True
        res = ListNode(0)
        head = res
        
        if l1 is None and l2 is None:
            return None
        elif l1 is None:
            return l2
        elif l2 is None:
            return l1
        
        while flag:            
            if l1.val < l2.val:
                res.next = l1
                l1 = l1.next
            else:
                res.next = l2
                l2 = l2.next
            
            res = res.next
            
            if l1 is None:
                res.next = l2
                flag = False
            elif l2 is None:
                res.next = l1
                flag = False
            
            
        return head.next

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