题目：链表倒数第n个节点

找到单链表倒数第n个节点，保证链表中节点的最少数量为n。

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样例

给出链表 3->2->1->5->null和n = 2，返回倒数第二个节点的值1.

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Cracking The Coding Interview 链表

/**
* 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 first node of linked list.
     * @param n: An integer.
     * @return: Nth to last node of a singly linked list.
     */
    ListNode nthToLast(ListNode head, int n) {
        // write your code here
        if(null==head) return null;
         ListNode x = head;
         int len = 1;
         while(x!=null){
              x = x.next;
              len++;
         }
         int count = len-n;
         ListNode u = head;
         while(count>1){
              u = u.next;
              count--;
         }
         return  u;
    }
}