Java版找链表的倒数第K个节点

思路：
1.给定的链表为空或者k<=0，返回空值
2.两个指针都指向链表的头节点
3.first指针先走到第k-1个节点上
4.两个链表同时向后遍历，first指针指向null时，last指针就是倒数第k个节点

class ListNode {
    int val;
    ListNode next = null;
    ListNode(int val) {
        this.val = val;
    }
}

public class Solution12 {
    public ListNode FindKthToTail(ListNode head, int k) {
        if (head == null || k <= 0) {
            return null;
        }
        ListNode firstNode = head;
        ListNode lastNode = head;
        for (int i = 1; i < k; i++) {
            if (firstNode.next != null) {
                firstNode = firstNode.next;
            } else {
                return null;
            }
        }
        while (firstNode.next != null) {
            firstNode = firstNode.next;
            lastNode = lastNode.next;
        }
        return lastNode;
    }
}