#include <ctype.h>
#include <stdio.h>

int atoi(char string[]) {
  int i, sign_ext, res;

  for (i = 0; isspace(string[i]); ++i)
    ;

  sign_ext = string[i] == '-' ? -1 : 1;
  if (string[i] == '-')
    ++i;

  for (res = 0; isdigit(string[i]); ++i) {
    res = res * 10 + (string[i] - '0');
  }

  return sign_ext * res;
}

int main() {
  printf("%d\n", atoi("123"));
  printf("%d\n", atoi("-123"));
  printf("%d\n", atoi("+123"));
  printf("%d\n", atoi("0123"));
  printf("%d\n", atoi("0123.1"));
}