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

  ListNode(int val) {
    this.val = val;
  }
}

public ListNode FindKthToTail(ListNode head,int k) {
    if(head == null || k <= 0){
      return null;
    }
    //直接遍历
    ListNode p = head;
    int length = 1;
    while(p.next != null){
      length++;
      p = p.next;
    }
    int index = length - k + 1;
    if(index <= 0){
      return null;
    }
    p = head;
    int num = 1;
    while(p.next != null && num < index){
      num++;
      p = p.next;
    }
    if(num < index){
      return null;
    }else{
      return p;
    }
  }

public ListNode FindKthToTail(ListNode head,int k) {
    if(head == null || k <= 0){
      return null;
    }
    //直接遍历
    ListNode p = head;
    ListNode q = head;
    for(int i = 0; i < k-1; i++){
      if(q == null){
        return null;
      }
      q = q.next;
    }
    if(q == null){
      return null;
    }
    while(q.next != null){
      p = p.next;
      q = q.next;
    }
    return p;
  }