面试题15：链表中倒数第K个结点

输入一个链表，输出该链表中倒数第k个结点。

方法1：

这个解法要循环两次链表

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

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution1 {
    public ListNode FindKthToTail(ListNode list,int k) {
        if (list == null)   return list;

        ListNode node = list;
        int count = 0;
        while (node != null) {
            count++;
            node = node.next;
        }
        if (count < k)  return null;

        ListNode p = list;
　　　　　//走n-k步
        for (int i = 0; i < count - k; i++) {
            p = p.next;
        }

        return p;
    }
}

方法2：快慢指针

/*
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 slow = head;
        ListNode fast = head;

        //fast指针先走k-1步
        for (int i=0;i<k-1;i++){
            if(fast.next==null){
                return null;
            }else {
                fast = fast.next;
            }
        }
        while (fast.next!=null){
            fast=fast.next;
            slow=slow.next;
        }
        return slow;
    }

}

可以参考单链表成环 https://www.cnblogs.com/chengpeng15/p/9868109.html