Print a long int in C using putchar() only

Write a C function print(n) that takes a long int number n as argument, and prints it on console. The only allowed library function is putchar(), no other function like itoa() or printf() is allowed. Use of loops is also not allowed.

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This is a simple trick question. Since putchar() prints a character, we need to call putchar() for all digits. Recursion can always be used to replace iteration, so using recursion we can print all digits one by one. Now the question is putchar() prints a character, how to print digits using putchar(). We need to convert every digit to its corresponding ASCII value, this can be done by using ASCII value of '0'. Following is complete C program.

/* C program to print a long int number
 using putchar() only*/
#include <stdio.h>

void print(long n)
{
    // If number is smaller than 0, put a - sign
    // and change number to positive
    if (n < 0) {
        putchar('-');
        n = -n;
    }

    // Remove the last digit and recur
    if (n/10)
        print(n/10);

    // Print the last digit
    putchar(n%10 + '0');
}

// Driver program to test above function
int main()
{
    long int n = 12045;
    print(n);
    return 0;
}

Output: 

12045

Time Complexity: O(logn)

Auxiliary Space: O(logn)

One important thing to note is the sequence of putchar() and recursive call print(n/10). Since the digits should be printed left to right, the recursive call must appear before putchar() (The rightmost digit should be printed at the end, all other digits must be printed before it).