Convert Hexadecimal String to Integer Without strtol or sscanf functions

Code

#include <stdio.h>
#include <stdint.h>
#include <string.h>

uint16_t hex_to_uint(const char *str) 
{
    int ans=0;
    int i=strlen(str)-1;
    int factor=1;
    int val;
    while(i>=0)
    {
        if(str[i]>='A' && str[i]<='F') 
           val=10+(str[i]-'A');
        
        if(str[i]>='a' && str[i]<='f')
           val=10+(str[i]-'a');

        if(str[i]>='1' && str[i]<='9')
           val=str[i]-'0';
      
      ans=ans+(val*factor);
      factor=factor*16;
      i--;
    }
    return ans;
}

int main() {
    char hex[10];
    scanf("%s", hex);

    printf("%u", hex_to_uint(hex));
    return 0;
}

Solving Approach