328. Odd Even Linked List

Medium

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

解題

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func oddEvenList(head *ListNode) *ListNode {
    if head == nil { return nil }
    
    odd := &ListNode{}
    oddH := odd
    even := &ListNode{}
    evenH := even
    flag := 1

    for head != nil {
        if flag == 1 {
            odd.Next = head
            odd = odd.Next
            flag = 0
        } else {
            even.Next = head
            even = even.Next
            flag = 1
        }

        head = head.Next
    }

    even.Next = nil
    odd.Next = evenH.Next

    return oddH.Next
}