方法：雙指標法 right指標先走K步，然後left，right指標一起走，當right = NULL，left指標到倒數第K個節點

#include <iostream>
#include <stack>
#include <vector>
using namespace std;

struct ListNode
{
public:
    int val;
    struct ListNode *next;
};

class Solution
{
public：

        // 14 連結串列中倒數第K個節點
        // 雙指標法
    
       
        ListNode* FindKthToTail(struct ListNode* pListHead, int k) {

            if (pListHead == NULL) {
                return NULL;
            }
            int i = 0;
            ListNode *right = pListHead;
            //right指標先向前走k步
            while (i < k && right != NULL) {
                right = right->next;
                i++;
            }

            if (right == NULL && i < k) {
                return NULL;
            }

            ListNode *left = pListHead;
            while (right != NULL) {
                left = left->next;
                right = right->next;
            }

            return left;
        }



};



int main()
{
    ListNode list[4];
    list[0].val = 1;
    list[0].next = &list[1];

    list[1].val = 2;
    list[1].next = &list[2];

    list[2].val = 3;
    list[2].next = &list[3];
    list[3].val = 4;
    list[3].next = NULL;

    Solution solu;
    
    ListNode* p = solu.FindKthToTail(list,3);
    cout << p->val << endl;

    
    return 0;
}