atoi

#include <iostream>

int atoi_shift(const char *p) {
    int k = 0;
    int sign = 1;

    if(p && p[0] == '-') {
        sign = -1;
        p++;
    }

    while (*p) {
        k = (k<<3)+(k<<1)+(*p)-'0';
        p++;
     }
     return sign*k;
}

int atoi_loop(const char *p)
{
    int k = 0;
    int sign = 1;

    if (p[0] == '-')
    {
        sign = -1;
        p++;
    }

    while(p) {
        k = k*10 + (*p) - '0';
    }

    return sign*k;
}

int main() {
    int a_l = atoi_loop("-1234");
    int a_s = atoi_shift("-1234");

    std::cout << a_l << '\n' << a_s << '\n';

    return 0;
}