题解 | #重排链表#

 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public void reorderList(ListNode head) {
        
        if(head == null){
            return;
        }
        ListNode slow = head;
        ListNode fast = head;
        while(fast.next != null && fast.next.next != null){
            slow = slow.next;
            fast = fast.next.next;
        }
        ListNode newHead= slow.next;
        slow.next = null;
        newHead = reverseNode(newHead);
        while(newHead != null){
            ListNode temp = newHead.next;
            newHead.next = head.next;
            head.next = newHead;
            head = newHead.next;
            newHead = temp;
        }
        
        
        
        
    }
    private ListNode reverseNode(ListNode tempHead){
        ListNode pre = null;
        ListNode currentNode = tempHead;
        while(currentNode != null){
            ListNode next = currentNode.next;
            currentNode.next = pre;
            pre = currentNode;
            currentNode = next;
        }
        return pre;
    }
}