剑指offer十四之链表中倒数第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 || k<=0){
            return null;
        }

        ListNode pre=head;
        ListNode last=head;

        for(int i=k-1;i>0;i--){ //移动k-1步，移动到第k个节点
            if(last.next!=null){
            last=last.next;
            }else {
                return null;
            }
        }

        while (last.next!=null){
            last=last.next;
            pre=pre.next;  //同步移动
        }
        return pre;
    }
}