연결된 리스트 뒤집기

206. Reverse Linked List

https://leetcode.com/problems/reverse-linked-list/

Given the head of a singly linked list, reverse the list, and return the reversed list.

Example 1:

Input: head = [1,2,3,4,5] Output: [5,4,3,2,1]

Example 2:

Input: head = [1,2] Output: [2,1]

Example 3:

Input: head = [] Output: []

Constraints:

• The number of nodes in the list is the range [0, 5000].
• -5000 <= Node.val <= 5000

Follow up: A linked list can be reversed either iteratively or recursively. Could you implement both?

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풀이 1. 재귀 구조로 뒤집기

def reverseList(self, head: ListNode) -> ListNode:
    def reverse(node: ListNode, prev: ListNode = None):
        if not node:
            return prev
        next, node.next = node.next, prev
        return reverse(next, node)
    
    return reverse(head)

풀이 2. 반복 구조로 뒤집기

def reverseList(self, head: ListNode) -> ListNode:
    node, prev = head, None
    
    while node:
        next, node.next = node.next, prev
        prev, node = node, next
    
    return prev

유효한 괄호

21. Merge Two Sorted Lists

https://leetcode.com/problems/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.

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풀이. 재귀 구조로 연결

def mergeTwoLists(self, l1:ListNode, l2: ListNode) -> ListNode:
    if (not l1) or (l2 and l1.val > l2.val):
        l1, l2 = l2, l1
    if l1:
        l1.next = self.mergeTwoLists(l1.next, l2)
    return l1