c program for the reverse the digits

Question

I am looking for the C program for reverse the digits like below:

If i enter:

123456

Then the result would

Answer 1

Here is a simple solution to this complex problem:

#include <stdio.h>

int main()
{
    int ch;
    ch = getchar();
    if (ch != '\n') {
        main();
        printf("%c", ch);
    }
}

---

A new version that outputs the newline:

#include <stdio.h>
#include <stdlib.h>

static void newline(void)
{
    printf("\n");
}

int main()
{
    int ch;
    ch = getchar();
    if (ch != '\n') {
        main();
        printf("%c", ch);
    } else {
        atexit(newline);
    }
}

Answer 2

If your professor is grading you on performance, then try ggf31416's solution to this problem in another question:

int FastReverse(int num) {
    int res = 0;
    int q = (int)((214748365L * num) >> 31);
    int rm = num - 10 * q;
    num = q;
    if (rm == 0) return -1;
    res = res * 10 + rm;
    while (num > 0) {
       q = (int)((214748365L * num) >> 31);
       rm = num - 10 * q;
       num = q;
       res = res * 10 + rm;
    }
    return res;
}

Optimization^®

^{When it absolutely, positively has to be done this nanosecond.}