编写一个函数，将一个数字字符串转换成该字符串对应的数字（包括正整数，负整数）

#include<stdio.h>
int my_atof(char *str)
{
int num = 0;
int result = 0;
int i;
if(str[0] == '-')
{
printf("-");
for(i = 1; str[i] != '\0';i++)
{
num = str[i] + (0 - '0');
result = result * 10 + num;
}
}
else
{
for(i = 0; str[i] != '\0';i++)
{
num = str[i] + (0 - '0');
result = result * 10 + num;
}
}
return result;
}

int main(void)
{
char *str1 = "12";
char *str2 = "-123";
printf("%d\n",my_atof(str1));
printf("%d\n",my_atof(str2));
return 0;
}