LeetCode #328 Odd Even Linked List

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Problem

Given a singly linked list, group all odd nodes together followed by the even nodes. Please note here we are talking about the node number and not the value in the nodes.

You should try to do it in place. The program should run in O(1) space complexity and O(nodes) time complexity.

Examples

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Input: 1->2->3->4->5->NULL
Output: 1->3->5->2->4->NULL
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Input: 2->1->3->5->6->4->7->NULL
Output: 2->3->6->7->1->5->4->NULL

Note

  • The relative order inside both the even and odd groups should remain as it was in the input.
  • The first node is considered odd, the second node even and so on …

Solution

Method: Iterative

Time Complexity: O(n)

Space Complexity: O(1)

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# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def oddEvenList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if not head:
            return head

        cur_odd = odd = head
        cur_even = even = head.next

        while cur_odd and cur_even:
            if cur_even.next:
                cur_odd.next = cur_even.next
                cur_odd = cur_odd.next
            else:
                break
                
            if cur_odd.next:
                cur_even.next = cur_odd.next
                cur_even = cur_even.next
            else:
                break

        cur_odd.next = even

        if cur_even:
            cur_even.next = None

        return head

or

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def oddEvenList(self, head):
    dummy1 = odd = ListNode(0)
    dummy2 = even = ListNode(0)
    while head:
        odd.next = head
        even.next = head.next
        odd = odd.next
        even = even.next
        head = head.next.next if even else None
    odd.next = dummy2.next
    return dummy1.next

Reference

https://leetcode.com/problems/odd-even-linked-list/discuss/78095/Clear-Python-Solution