剑指offer---反转链表

问题：反转链表

要求：输入一个链表，反转链表后，输出新链表的表头。

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
            val(x), next(NULL) {
    }
};*/
class Solution {
public:
    ListNode* ReverseList(ListNode* pHead) {

    }
};

对于翻转链表的解法，在博客链表ADT实现中已经完成，只是将其定义为了链表的一种方法，将其代码稍加修改就可以作为此题的解答，代码如下：

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

class Solution {
public:
    ListNode* ReverseList(ListNode* pHead) {
        if (pHead == nullptr || pHead->next == nullptr)
            return pHead;
        ListNode *pre = pHead;
        ListNode *cur = pHead->next;
        ListNode *temp = pHead->next->next;
        while (cur){
            temp = cur->next;
            cur->next = pre;
            pre = cur;
            cur = temp;
        }
        // 末尾元素指针置空
        pHead->next = nullptr;
        // 头指针
        ListNode *newHead = pre;
        return newHead;
    }
};

代码验证：

若原链表为1(100)->2(200)->3(300)，括号中的数字为节点的地址。将原链表反转，则反转之后 head = 300; 

以下为验证代码，代码先输出链表，紧接着下一行输出链表每个元素的地址。 

#include<iostream>
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(); // 链表长度
    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
    void reverse();
};

// 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;

    p = head;
    while(p != NULL){
        cout<<p<<" ";
        p = p->next;
    }
    cout<<endl<<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);
}

// 反转链表
void List::reverse(){
    if (head == NULL || head->next == NULL)
        return ;
    Node *prev = head;
    Node *cur = head->next;
    Node *temp = head->next->next;
    while(cur){
        temp = cur->next;
        cur->next = prev;
        prev = cur;
        cur = temp;
    }
    head->next = NULL; // 更新末尾元素的指针
    head = prev; // 更新头结点
}

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);
    cout<<endl<<"The list is:"<<endl;
    l1.printList();
    l1.reverse();
    cout<<endl<<"The reversed list is:"<<endl;
    l1.printList();
    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 
0x661a70 0x661a50 0x661a30 0x661a60 0x661a40 0x661a20 0x661a80 


The reversed list is:
16 6 5 56 7 16 169 
0x661a80 0x661a20 0x661a40 0x661a60 0x661a30 0x661a50 0x661a70 

The list is deleted.

可见，地址依次倒序，说明代码正确。