编程实现字符串的替换

问题
用C++写一个小程序，先请用户输入3个字符串，然后把在第一个字符串中出现的所有第2个字符串替换成第3个字符串，最后输出新的字符串；
代码

#include <iostream>
using namespace std;
char *replace(const char *str,const char *sub1,const char *sub2,char *output)
{
    char *pOutput = NULL;
    const char *pStr = NULL;
    int lensub1 = strlen(sub1);
    int lensub2 = strlen(sub2);
    pOutput = output;
    pStr = str;
    while(*pStr!=0)
    {
        pStr = strstr(pStr,sub1);//在str中寻找sub1子串
        if(NULL!= pStr)   //找到sub1子串
        {
            memcpy(pOutput,str,pStr-str);
            pOutput+=pStr-str;
            memcpy(pOutput,sub2,lensub2);
            pOutput+=lensub2;
            pStr += lensub1;
            str = pStr;
        }
        else
        {
            break;
        }
    }
    *pOutput ='\0';
    if(*str != '\0')
    {
        strcpy(pOutput,str);
    }
    return output;
}
int main()
{
    char str[50] ="";
    char sub1[10] = "";
    char sub2[10] = "";
    char output[100] = "";
    cout<<"str: ";
    cin>>str;
    cout<<"sub1: ";
    cin>>sub1;
    cout<<"sub2: ";
    cin>>sub2;
    cout<<replace(str,sub1,sub2,output)<<endl;
    return 0;
}