odd even linked list

Author: always-maap

328.OddEvenLinkedList.py

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+"""
+328. Odd Even Linked List
+Medium
+Linked List
+---
+Given the head of a singly linked list, group all the nodes with 
+ odd indices together followed by the nodes with even indices, and return the reordered list.
+
+The first node is considered odd, and the second node is even, and so on.
+
+Note that the relative order inside both the even and odd groups should remain as it was in the input.
+
+You must solve the problem in O(1) extra space complexity and O(n) time complexity.
+
+Example 1:
+Input: head = [1,2,3,4,5]
+Output: [1,3,5,2,4]
+
+Example 2:
+Input: head = [2,1,3,5,6,4,7]
+Output: [2,3,6,7,1,5,4]
+
+Constraints:
+The number of nodes in the linked list is in the range [0, 104].
+-106 <= Node.val <= 106
+"""
+
+
+from typing import Optional
+
+
+# Definition for singly-linked list.
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+
+
+# O(n) time | O(1) space
+class Solution:
+    def oddEvenList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if head:
+            odd = head
+            even = even_head = head.next
+            while even and even.next:
+                odd.next = odd = even.next
+                even.next = even = odd.next
+            odd.next = even_head
+            return head