35 - 翻转链表

4.18

一遇到链表的问题就化身成为一个懵蛋，搞不懂谁指向谁。

/**
 * Definition for ListNode.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int val) {
 *         this.val = val;
 *         this.next = null;
 *     }
 * }
 */ 
public class Solution {
    /**
     * @param head: The head of linked list.
     * @return: The new head of reversed linked list.
     */
    public ListNode reverse(ListNode head) {
        if(head == null || head.next == null){
            return head;
        }// write your code here
        ListNode tmp = head.next;
        head.next = null;
        //ListNode flag = head;
        while(tmp != null){
            ListNode flag = tmp.next;
            tmp.next = head;
            head = tmp;
            tmp = flag;
            
        }
        return head;
    }
}