119. Convert Hexadecimal String to Integer Without strtol or sscanf functions

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

uint16_t hex_to_uint(const char *str)
{
    uint16_t res = 0;
    int i = 0;

    while (str[i] != '\0')
    {
        res <<= 4;  // nh&#226;n 16
        if (str[i] >= '0' && str[i] <= '9')
            res += (str[i] - '0');
        else if (str[i] >= 'A' && str[i] <= 'F')
            res += (str[i] - 'A' + 10);
        else if (str[i] >= 'a' && str[i] <= 'f')
            res += (str[i] - 'a' + 10);
        else
            return 0; 
        ++i;
    }

    return res;
}

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

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