牛客_链表的奇偶重排

链表的奇偶重排
https://www.nowcoder.com/practice/02bf49ea45cd486daa031614f9bd6fc3

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

/**
 * NC133 链表的奇偶重排
 * @author d3y1
 */
public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param head ListNode类
     * @return ListNode类
     */
    public ListNode oddEvenList (ListNode head) {
        return solution(head);
    }

    /**
     * 链表排序: 奇偶节点
     * @param head
     * @return
     */
    private ListNode solution(ListNode head){
        if(head == null){
            return null;
        }

        ListNode oddHead = head;
        ListNode oddTail = head;
        ListNode evenHead = head.next;
        ListNode evenTail = head.next;

        // 关键
        while(evenTail!=null && evenTail.next!=null){
            oddTail.next = evenTail.next;
            oddTail = oddTail.next;
            evenTail.next = oddTail.next;
            evenTail = evenTail.next;
        }

        // if(evenHead != null){
        //     oddTail.next = evenHead;
        // }
        oddTail.next = evenHead;

        return oddHead;
    }
}