剑指offer 链表中倒数第k个结点

题目描述：

输入一个链表，输出该链表中倒数第k个结点。

(hint: 请务必使用链表。)

思路：

单向链表，故不能倒着来，我们可以定义两个指针p1，p2，对于倒数第k个，我们先令p1走k步，然后两个指针一起走，如果p1到了结尾，则p2所对应的值就是所求。

#include <cstdio>
using namespace std;

typedef struct Node{
int val;
Node *next;
Node(int val = -1) : val(val), next(NULL) {}
}*pNode;

class List{
private:
pNode Head;
pNode Tail;
int List_Size;
void Delete_List(pNode);
public:
void Delete_Content();
int Find(int x);
void Insert_Node(int x);
List();
~List();
};

void List::Insert_Node(int x){
Node *node = new Node(x);
List_Size++;
this->Tail->next = node;
this->Tail = node;
}

int List::Find(int x){
if(x <= 0 || x > List_Size)
return -1;
pNode p1 = this->Head->next, p2 = this->Head->next;
for(int i = 0; i < x; i++)
p1 = p1->next;
while(p1){
p1 = p1->next;
p2 = p2->next;
}
return p2->val;
}

void List::Delete_Content(){
pNode p = this->Head->next;
while(p){
pNode child = p->next;
delete p;
p = child;
}
this->Head->next = NULL;
this->Tail = this->Head;
List_Size = 0;
}

void List::Delete_List(pNode node){
if(node){
Delete_List(node->next);
delete node;
}
}

List::List(){
Head = Tail = new Node();
List_Size = 0;
}

List::~List(){
Delete_List(this->Head);
}

int main()
{
int n, x;
int v;
List *list = new List;
while(scanf("%d%d", &n, &x) != EOF){
for(int i = 0; i < n; i++){
scanf("%d", &v);
list->Insert_Node(v);
}
int ans = list->Find(x);
if(ans == -1)
printf("NULL\n");
else
printf("%d\n", ans);
list->Delete_Content();
}
delete list;
return 0;
}