atoi的实现

//字符串到整数
int atoi1(char *str)
{
int sign = 1, num = 0;
if (*str == '-')
{
sign = -1;
str++;
}

while (*str != '\0')
{
num = num * 10 + (*str - '0');   //字符相减是值，比如a = 3;b = 4;c = a - b;那么 c = -1;字符就是一个“代词”而已
str++;
}
return num * sign;
}