my_atoi

#include<iostream>
using namespace std;
int my_atoi(char *s)
{
    if(s==NULL) 
        return -1;
    char *p=s;
    int n=0,sum=0;
    while(*p++)
        n++;
    bool isNegative=false;
    int j=0;
    if(s[0]=='-')
    {
        isNegative=true;
        j=1;
    }
    for(;j<n;j++)
        sum=sum*10+s[j]-'0';
    if(isNegative)
        sum*=-1;
    return sum;
}
int main()
{
    char *s="-123";
    cout<<my_atoi(s)<<endl;
    system("pause");
    return 0;
}