C Program to Print Number series without using any loop

Write a C program for given two number N and K, our task is to subtract a number K from N until number(N) is greater than zero, once the N becomes negative or zero then we start adding K until that number become the original number(N).

Note: Not allow to use any loop.

Examples :

Input : N = 15 K = 5
Output : 15 10 5 0 1 5 10 15

Input : N = 20 K = 6
Output : 20 14 8 2 -4 2 8 14 20

C Program to Print Number series without using any loop using Recursion:

We can do it using recursion idea is that we call the function again and again until N is greater than zero (in every function call we subtract N by K). Once the number becomes negative or zero we start adding K in every function call until the number becomes the original number. Here we use a single function for both addition and subtraction but to switch between addition or subtraction function we used a Boolean flag. 

Below is the implementation of the above approach:

// C Program for the above approach
#include <stdio.h>

// Function to print the number series using recursion
void PrintNumber(int N, int Original, int K, int flag)
{
    // Print the number
    printf("%d ", N);

    // Change flag if the number becomes negative
    if (N <= 0)
        flag = !flag;

    // Base condition for the second case (Adding K)
    if (N == Original && !flag)
        return;

    // If the flag is true, subtract the value until the
    // number is greater than zero
    if (flag) {
        PrintNumber(N - K, Original, K, flag);
        return;
    }

    // Second case (Addition)
    if (!flag) {
        PrintNumber(N + K, Original, K, flag);
        return;
    }
}

// Driver Code
int main()
{
    int N = 20, K = 6;

    PrintNumber(N, N, K, 1);

    return 0;
}

20 14 8 2 -4 2 8 14 20 

Time Complexity: O(N), where N is the value of the N variable
Auxiliary Space: O(N)

Please refer complete article on Print Number series without using any loop for more details!