328. 奇偶链表

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func oddEvenList(head *ListNode) *ListNode {
    if head == nil {
        return head
    }
    oddHead := &ListNode{ Val: head.Val, Next: nil }
    head = head.Next
    evenFirst := head
    oddFirst := oddHead
    for head != nil {
        oddHead.Next = head.Next
        if oddHead.Next != nil {
            head.Next = oddHead.Next.Next
            oddHead = oddHead.Next
        }
        head = head.Next
    }
    oddHead.Next = evenFirst
    return oddFirst
}