Convert Hexadecimal String to Integer Without strtol or sscanf functions

Code

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

uint16_t hex_to_uint(const char *str) {\
    int len=0;
    uint16_t result;
    int factor = 1;
    int hexbit;

    while (str[len] != '\0') {
        len++;
    }

    for (int i = len-1; i>=0; i--) {
        if ((str[i] >= '0') && (str[i] <= '9')) {
            hexbit = str[i] - '0'; 
        } else if ((str[i] >= 'A') && (str[i] <= 'F')) {
            hexbit = str[i] - 'A' + 10;
        } else if ((str[i] >= 'a') && (str[i] <= 'f')) {
            hexbit = str[i] - 'a' + 10;
        }
        result += hexbit*factor;
        factor *= 16;
    }

    return result;
}

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

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

Solving Approach