class Solution {
public:
    ListNode* FindKthToTail(ListNode* pListHead, unsigned int k) {
        ListNode *fast=pListHead;
        ListNode *slow=pListHead;
        while(k--){
            if(fast)
                fast=fast->next;
            else
                return NULL;
        }
        while(fast){
            slow=slow->next;
            fast=fast->next;
        }
        return slow;
    }
};