剑指offer-8：链表中倒数第k个结点

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode FindKthToTail(ListNode head,int k) {
        if(head==null){
            return null;
        }
        ListNode p1 = head,p2 = head;
        while(k > 0 & p2 != null){
            p2 = p2.next;
            k--;
        }
        //p2 指向head说明k<=0,p2==null && k>0说明 k超过了链表的长度
        if(p2 == head || (p2==null&&k>0)){
            return null;
        }
        while(p2 != null){
            p1 = p1.next;
            p2 = p2.next;
        }
        return p1;
    }
}