问题描述

以一个链表指针作参数传入一个函数里，若此时链表为空，则初始化链表，为什么在主函数中不能用？

#include
using namespace std;

typedef struct Node
{
int age;
struct Node* next;
}LIST;

void addList1(LIST* pHead,int x)//链表结尾加节点
{
LIST* pM=new LIST();//新节点
pM->age=x;
pM->next=NULL;
if(pHead==NULL)
{
cout<<"List is NULL"<
cout
pHead=new LIST();//重新分配内存，头节点
pHead->age=0;
pHead->next=pM;
}
else
{
LIST* pCur=pHead->next;//当前节点
while(pCur!=NULL)
{
pCur=pCur->next;
}
pCur=pM;
}
}
LIST* CreatList1()//创建节点
{
int data=0;
LIST* Phead=NULL;
LIST* Pm=NULL;
LIST* Pcur=NULL;
cout<<"Enter your data of node (-1 quit):";
scanf("%d", &data);
if(data!=-1)
{
Phead=(LIST*)malloc(sizeof(LIST));
Phead->age=0;
Phead->next=NULL;
Pcur=Phead;
}
while (data!=-1)
{
Pm=new LIST();
Pm->age=data;
Pm->next=NULL;
Pcur->next=Pm;
Pcur=Pcur->next;
cout<<"Enter your data of node (-1 quit):";
cin>>data;
}
return Phead;
}

void ListOut1(LIST* Phead)//输出节点
{
LIST* p=NULL;

if(Phead==NULL)
{
    cout<<"List is NULLn";
}
else
{
    p=Phead->next;
    while(p!=NULL)
    {
        cout<<p->age<<endl;
        p=p->next;
    }
}

}

void main()
{
LIST* p1=CreatList1();
ListOut1(p1);
addList1(p1,100); //当输入链表为空时，初始化链表
ListOut1(p1); //此时输出为空？ 为什么？ 我不是在初始化时在堆中分配的内存 不是可以用吗？
system("pause");
}

解决方案

 // ConsoleApplication2.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"

#include <iostream>
using namespace std;

typedef struct Node
{
    int age;
    struct Node* next;
}LIST;

void addList1(LIST*& pHead, int x)//链表结尾加节点
{
    LIST* pM = new LIST();//新节点
    pM->age = x;
    pM->next = NULL;
    if (pHead == NULL)
    {
        cout << "List is NULL" << endl;
        cout << "Creat New List" << endl;
        pHead = new LIST();//重新分配内存，头节点
        pHead->age = 0;
        pHead->next = pM;
    }
    else
    {
        if (pHead->next == NULL)
        {
            pHead->age = x;
        }
        else
        {
            LIST* pCur = pHead->next;//当前节点
            while (pCur->next != NULL)
            {
                pCur = pCur->next;
            }
            pCur->next = pM;
        }
    }
}

LIST* CreatList1()//创建节点
{
    int data = 0;
    LIST* Phead = NULL;
    LIST* Pm = NULL;
    LIST* Pcur = NULL;
    cout << "Enter your data of node (-1 quit):";
    scanf_s("%d", &data);
    if (data != -1)
    {
        Phead = (LIST*)malloc(sizeof(LIST));
        Phead->age = 0;
        Phead->next = NULL;
        Pcur = Phead;
    }
    while (data != -1)
    {
        Pm = new LIST();
        Pm->age = data;
        Pm->next = NULL;
        Pcur->next = Pm;
        Pcur = Pcur->next;
        cout << "Enter your data of node (-1 quit):";
        cin >> data;
    }
    return Phead;
}

void ListOut1(LIST* Phead)//输出节点
{
    LIST* p = NULL;

    if (Phead == NULL)
    {
        cout << "List is NULLn";
    }
    else
    {
        p = Phead->next;
        while (p != NULL)
        {
            cout << p->age << endl;
            p = p->next;
        }
    }

}

void main()
{
    LIST* p1 = CreatList1();
    ListOut1(p1);
    addList1(p1, 100); //当输入链表为空时，初始化链表
    ListOut1(p1);      //此时输出为空？  为什么？  我不是在初始化时在堆中分配的内存  不是可以用吗？
    system("pause");
}

解决方案二：

 void addList1(LIST* pHead, int x)//链表结尾加节点
{
    LIST* pM = new LIST();//新节点
    pM->age = x;
    pM->next = NULL;
    if (pHead == NULL)
    {
        cout << "List is NULL" << endl;
        cout << "Creat New List" << endl;
        pHead = new LIST();//重新分配内存，头节点
        pHead->age = 0;
        pHead->next = pM;
    }
    else
    {
        LIST* pCur = pHead->next;//当前节点
        while (pCur->next != NULL)
        {
            pCur = pCur->next;
        }
        pCur->next = pM;
    }
}

解决方案三：

你的程序没有问题，只是添加节点不对，节点没有被添加到链表上。我修改了下

 Enter your data of node (-1 quit):1
Enter your data of node (-1 quit):2
Enter your data of node (-1 quit):3
Enter your data of node (-1 quit):4
Enter your data of node (-1 quit):5
Enter your data of node (-1 quit):6
Enter your data of node (-1 quit):-1
1
2
3
4
5
6
1
2
3
4
5
6
100
Press any key to continue . . .

解决方案四：

应该是传参有问题吧。应该是这样listadd1(&p1,100)

解决方案五：

主要是你在addList1()里面的“cout<<"List is NULL"< cout pHead=new LIST();//重新分配内存，头节点”，这里改变的pHead是局部变量，所以不会对主函数里的p1造成影响。如果想在子函数里改变指针，那么就要用2级指针把指针的地址传进去。

解决方案六：

用new分配的内存如果没有用delete收不是一直存在吗？ 难道是因为在被调用函数里重新定义的链表，所以在主调函数里不能使用？