Convert String to Integer

Code

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

int custom_atoi(const char *str) {
    int i=0;
    int flag = 1;
    int res  = 0;

    if(*str == '-'){
        flag = -1;
        str++;
    }else if(*str == '+'){
        flag = 1;
        str++;
    }else if(str[0] >= 'a' && str[0] <= 'z'){
        return res;
    }

    while(*str){
        if(*str < '0' || *str > '9'){
            return flag*res;
        }else{
            res = res*10 + (*str - '0');
        }
        str++;
    }

    return flag * res;
}

int main() {
    char str[101];
    fgets(str, sizeof(str), stdin);

    // Remove newline
    uint8_t i = 0;
    while (str[i]) {
        if (str[i] == '\n') {
            str[i] = '\0';
            break;
        }
        i++;
    }

    printf("%d", custom_atoi(str));
    return 0;
}

Solving Approach