[코테] 21. Merge Two Sorted Lists

난이도:EasySolved

My Solution

# 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, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
        # 가짜 노드와 현재 포인터 생성
        dummy = ListNode()
        current = dummy
				
				# .val은 해당 노드에 있는 값을 의미
        while list1 and list2:
            if list1.val < list2.val:
                current.next = list1
                list1 = list1.next
            else:
                current.next = list2
                list2 = list2.next
            
            current = current.next
        if list1 or list2:
            current.next = list1 if list1 else list2
        return dummy.next

Other Solution

# recursively    
def mergeTwoLists2(self, l1, l2):
    if not l1 or not l2:
        return l1 or l2
    if l1.val < l2.val:
        l1.next = self.mergeTwoLists(l1.next, l2)
        return l1
    else:
        l2.next = self.mergeTwoLists(l1, l2.next)
        return l2
        
# in-place, iteratively        
def mergeTwoLists(self, l1, l2):
    if None in (l1, l2):
        return l1 or l2
    dummy = cur = ListNode(0)
    dummy.next = l1
    while l1 and l2:
        if l1.val < l2.val:
            l1 = l1.next
        else:
            nxt = cur.next
            cur.next = l2
            tmp = l2.next
            l2.next = nxt
            l2 = tmp
        cur = cur.next
    cur.next = l1 or l2
    return dummy.next