/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* findKthToTail(ListNode* head, int k) {
        //双指针，也是考研算法题，注意边界
        if(!head) return nullptr;

        auto p = head,q = head;
        while(k--){
            if(q) q = q->next;
            else return nullptr;
        }

        while(q) p = p->next,q = q->next;
        return p;
    }
};