solve 2

Author: lylaminju

reorder-list/pmjuu.py

@@ -0,0 +1,37 @@
+'''
+시간 복잡도: O(n)
+- 중간 노드 찾기: O(n)
+- 리스트 반전: O(n)
+- 리스트 병합: O(n)
+
+공간 복잡도: O(1)
+- 포인터만 사용하여 링크를 조작하므로 O(1)
+'''
+from typing import Optional
+# Definition for singly-linked list.
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+        
+class Solution:
+    def reorderList(self, head: Optional[ListNode]) -> None:
+        """
+        Do not return anything, modify head in-place instead.
+        """
+        # find the middle node
+        slow, fast = head, head
+        while fast and fast.next:
+            slow = slow.next
+            fast = fast.next.next
+
+        # reverse back half of the list
+        prev, curr = None, slow.next
+        slow.next = None # cut in the middle
+        while curr:
+            curr.next, prev, curr = prev, curr, curr.next
+
+        # merge front half with back half
+        first, second = head, prev
+        while second:
+            first.next, second.next, first, second = second, first.next, first.next, second.next