Linked list - the penultimate K nodes

subject ：

Enter a length of n The linked list of , Set the value of the element in the linked list to ai , Returns the penultimate... In the linked list k Nodes .
If the length of the list is less than k, Please return a length of 0 The linked list of .
For example, the input {1,2,3,4,5},2 when , The corresponding linked list structure is shown in the figure below ：

The blue part is the last part of the linked list 2 Nodes , So return to the penultimate 2 Nodes （ That is, the node value is 4 The node of ） that will do , The system will print all the following nodes to compare .

Their thinking ：

Set two pointers ,flag and cur,flag When you reach the end of the linked list ,flag And cur Distance identification between k value .

Solution code ：

/**
 * struct ListNode {
 *	int val;
 *	struct ListNode *next;
 *	ListNode(int x) : val(x), next(nullptr) {}
 * };
 */
class Solution {
public:
    /**
     *  The class name in the code 、 Method name 、 The parameter name has been specified , Do not modify , Return the value specified by the method directly 
     *
     * 
     * @param pHead ListNode class  
     * @param k int integer  
     * @return ListNode class 
     */
    ListNode* FindKthToTail(ListNode* pHead, int k) {
        // write code here
        ListNode* flag=pHead;
        ListNode* cur=pHead;
        for(int i=0;i<k;i++){
            if(!flag){
                return nullptr;// When flag The value is null When , Express k Value is greater than the length of the linked list 
            }
            flag=flag->next;
        }
        while(flag){
            cur=cur->next;
            flag=flag->next;
        }
        return cur;
    }
};