Reorder List

将midnode后面的node的指针反转，再从头尾两边同时遍历

注意midnode的next设为null

public class Solution {
    public void reorderList(ListNode head) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        int count = 0;
        ListNode midnode = head;
        ListNode runner = head;
        while(runner != null&&runner.next!=null)
        {
            runner = runner.next.next;
            midnode = midnode.next;
            count++;
        }
        if(count < 1||midnode.next == null)
            return;
        
        ListNode fastrunner = midnode.next;
        runner = midnode;
        while(fastrunner!=null)
        {
            ListNode tmp = fastrunner.next;
            fastrunner.next = runner;
            runner = fastrunner;
            fastrunner = tmp;
        }
        ListNode frontrunner = head;
        ListNode backrunner = runner;
        while(frontrunner!=midnode && backrunner!=midnode)
        {
            ListNode tmp = backrunner.next;
            backrunner.next = frontrunner.next;
            frontrunner.next = backrunner;
            frontrunner = frontrunner.next.next;
            backrunner = tmp;
        }
        midnode.next = null;
    }
    
}