【剑指offer 面试题15】链表中倒数第K个结点

思路：

　　定义两个指针同时指向head，第一个指针先走K-1步，随后二个指针同时移动，当第一个指针到末尾处时，第二个指针所指向的即为倒数第K个结点。

 #include <iostream>
 using namespace std;

 struct ListNode
 {
     int val;
     ListNode *next;
     ListNode(int v = ):val(v), next(NULL){}
 };

 ListNode *findLastKthnumber(ListNode **pListHead, unsigned int k)
 {
     if(*pListHead == NULL || k == )
         return NULL;

     ListNode *pFirst = *pListHead;
     ListNode *pSecond = *pListHead;

     for(int i = ; i < k - ; i++)
     {
         pFirst = pFirst->next;
         if(pFirst == NULL)
             return NULL;
     }

     while(pFirst->next != NULL)
     {
         pFirst = pFirst->next;
         pSecond = pSecond->next;
     }

     return pSecond;
 }

 int main()
 {
     ListNode *first = new ListNode();
     ListNode *head = first;

     ListNode *second = new ListNode();
     first->next = second;
     first = first->next;

     ListNode *third = new ListNode();
     first->next = third;
     first = first->next;

     cout<<"初始链表： ";
     ListNode *print = head;
     while(print != NULL)
     {
         cout<<print->val<<"  ";
         print = print->next;
     }
     cout<<endl<<endl;

     cout<<"倒数第零个结点： ";
     ListNode *res = findLastKthnumber(&head, );
     if(res != NULL)
         cout<<res->val<<endl;
     else
         cerr<<"error!"<<endl;

     cout<<"倒数第一个结点： ";
     res = findLastKthnumber(&head, );
     if(res != NULL)
         cout<<res->val<<endl;
     else
         cerr<<"error!"<<endl;

     cout<<"倒数第二个结点： ";
     res = findLastKthnumber(&head, );
     if(res != NULL)
         cout<<res->val<<endl;
     else
         cerr<<"error!"<<endl;

     cout<<"倒数第三个结点： ";
     res = findLastKthnumber(&head, );
     if(res != NULL)
         cout<<res->val<<endl;
     else
         cerr<<"error!"<<endl;

     cout<<"倒数第四个结点： ";
     res = findLastKthnumber(&head, );
     if(res != NULL)
         cout<<res->val<<endl;
     else
         cerr<<"error!"<<endl;
 }

测试结果：

初始链表：     

倒数第零个结点： error!
倒数第一个结点：
倒数第二个结点：
倒数第三个结点：
倒数第四个结点： error!