Convert string to integer num, convert integer num to string

Convert string to integer num, convert integer num to string
1 A
2 B
…
27 AA
28 AB
int foo(char* str)
{
if(!str) return 0;
int num=0;
int len=strlen(str)-1;
int idx=len;
while(idx>=0)
{
num+=(str[idx]-'A'+1)*pow(26,len-idx);
idx--;
}
return num;
}
char* foo(int num)
{
if(num<=0) return NULL;
char* str=new char[11];
int idx=0;
while(num>=0)
{
str[idx]=num%26+'A'-1;
num/=26;
idx++;
}
str[idx]='\0';
return reverse(str);
}