剑指offer---从尾到头打印链表

问题：剑指offer---从尾到头打印链表

要求：输入一个链表，按链表值从尾到头的顺序返回一个ArrayList。

/**
*  struct ListNode {
*        int val;
*        struct ListNode *next;
*        ListNode(int x) :
*              val(x), next(NULL) {
*        }
*  };
*/
class Solution {
public:
    vector<int> printListFromTailToHead(ListNode* head) {
        
    }
};

在上一篇博客中已经建立了链表类，基于上述基础进行适当修改即可解决上述问题。

方法一：采用链表类反转链表方法的思想，采用三指针法将链表指针倒序（但是此方法改变了链表的结构）；

方法二：先将链表从头到尾遍历，存到堆栈中，利用堆栈后进先出的特点，依次弹出就可。（没有对原链表进行改动，本文采用方法二）。

大部分代码与上篇博客中的链表类相同，只是增加了一个 printListFromTailToHead(ListNode* head) 函数，同时把 class List() 的 Node *head; 改为了 public 

其中 printListFromTailToHead(ListNode* head) 函数如下：

vector<int> printListFromTailToHead(Node* head){
    stack<int> temp;
    Node *p = head;
    while (p != nullptr){
        temp.push((*p).data);
        p = p->next;
    }
    vector<int> res;
    while (!temp.empty()){
        res.push_back(temp.top());
        temp.pop();
    }
    return res;
}

完整代码

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

class Node {
public:
    int data;
    Node *next;
    Node(int da):
        data(da), next(NULL){}
};

class List{
public:
    Node *head;
    List(): head(NULL){}
    ~List(){
        delete head;
        cout<<"The list is deleted."<<endl;
    };
    int size();
    void printList(); // 打印链表
    void insert(int position, int value); // 指定位置插入
    void insertHead(int value); // 插入到最前
    void insertTail(int value); // 插入到最后
};

// 返回链表大小
int List::size(){
    Node *p = head;
    int index = 0;
    while(p != NULL){
        index++;
        p = p->next;
    }
    return index;
}

// 打印链表
void List::printList(){
    Node *p = head;
    while(p != NULL){
        cout<<p->data<<" ";
        p = p->next;
    }
    cout<<endl;
    cout<<endl;
}

// 在position位置插入value
void List::insert(int position, int value){
    if(position<0 || position>List::size()){
        cout<<"position error, please check."<<endl;
        return ;
    }
    Node *s = new Node(value); // new node
    Node *p = head;
    if(head == NULL){ // isEmpty = true
        head = s;
    }
    else{ // isEmpty = false
        if(position == 0){
            s->next = p;
            head = s;
        }
        else{
            int index = 0;
            while(index != position-1){
                p = p->next;
                index++;
            }
            s->next = p->next;
            p->next = s;
        }
    }
    if (position == 0)
        cout<<"insert "<<value<<" at the first."<<endl;
    else if (position == List::size())
        cout<<"insert "<<value<<" at the tail."<<endl;
    else
        cout<<"insert "<<value<<" at position "<<position<<endl;
}

// 头部插入
void List::insertHead(int value){
    List::insert(0, value);
}

// 尾部插入
void List::insertTail(int value){
    List::insert(List::size(), value);
}

// printListFromTailToHead
vector<int> printListFromTailToHead(Node* head){
    stack<int> temp;
    Node *p = head;
    while (p != nullptr){
        temp.push((*p).data);
        p = p->next;
    }
    vector<int> res;
    while (!temp.empty()){
        res.push_back(temp.top());
        temp.pop();
    }
    return res;
}

int main() {
    List l1;
    l1.insertTail(6);
    l1.insertHead(7);
    l1.insert(1, 5);
    l1.insert(0, 16);
    l1.insert(2, 56);
    l1.insert(0, 169);
    l1.insert(6, 16);

    vector<int> result;
    Node *p = l1.head;
    result = printListFromTailToHead(p);
    cout<<endl<<"The list is:"<<endl;
    l1.printList();
    cout<<"print list from tail to head:"<<endl;
    for(auto &it : result)
        cout<<it<<' ';
    cout<<endl<<endl;
    return 0;
}

运行结果

insert 6 at the first.
insert 7 at the first.
insert 5 at position 1
insert 16 at the first.
insert 56 at position 2
insert 169 at the first.
insert 16 at position 6

The list is:
169 16 7 56 5 6 16 

print list from tail to head:
16 6 5 56 7 16 169 

The list is deleted.
[Finished in 2.6s]