21. Merge Two Sorted Lists

# Easy

注意dummy node的用法

Python

class Solution:
    def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
        # edge case
        if l1 == None and l2 != None:
            return l2
        if l1 != None and l2 == None:
            return l1
        if l1 == None and l2 == None:
            return l1
        
        dummy = ListNode(0)
        head = dummy
            
        while l1 != None and l2 != None:
            if l1.val <= l2.val:
                head.next = l1
                l1 = l1.next
            else:
                head.next = l2
                l2 = l2.next
            head = head.next
            
        if l1 != None:
            head.next = l1
        elif l2 != None:
            head.next = l2
            
        return dummy.next

Second Tab