[leetcode] 21. Merge Two Sorted Lists

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]

 

 

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def mergeTwoLists(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
        l3 = []
        while (l1 and l2) : # 두 ListNode가 모두 빈 리스트 여야 while 문 멈춤 
            # print(l1.val)
            # print(l2.val)
            if l1.val <= l2.val:
                l3.append(l1.val) 
                l1 = l1.next 
            else:
                l3.append(l2.val)
                l2 = l2.next
        
        if l1 is not None:
            t = l1 
        else:
            t = l2
        while t:
            l3.append(t.val)
            t = t.next
            
        print(l3)

        head = ListNode(None)
        node = head 
        while l3: 
            a = l3.pop(0)
            one = ListNode(a)
            node.next = one
            node = one

        return head.next # 맨 처음껄 리턴해야지 중간꺼 보는거 아님!