atoi() 函数的逆过程 itoa()

 /* itoa:  convert n to characters in s */
 void itoa(int n, char s[])
 {
     int i, sign;
     if ((sign = n) < 0)  /* record sign */
         n = -n;          /* make n positive */
     i = 0;
     do {      /* generate digits in reverse order */
         s[i++] = n % 10 + '0';  /* get next digit */
     } while ((n /= 10) > 0);    /* delete it */
     if (sign < 0)
         s[i++] = '-';
     s[i] = '\0';
     reverse(s);
 }