Print 1 to n

Problem Link

Print 1 to N Without Using Loops – GeeksforGeeks

Problem: Print 1 to N Without Using Loops

Problem Statement

Given a positive integer N, print numbers from 1 to N using recursion only. You are not allowed to use loops (for, while, etc.).

Examples

Example 1
Input: N = 5
Output: 1 2 3 4 5

Example 2
Input: N = 3
Output: 1 2 3

Constraints

• 1 <= N <= 10⁴

Intuition

We are asked to simulate a loop behavior using recursion.
Since we want to print numbers in increasing order from 1 to N, we should begin at 1 and recursively call the next number until we reach N.

This is a pre-order style recursion.

Approach

1. Define a recursive function printNos(int i, int N).

2. Base case: if i > N, return.

3. Print the value i.

4. Recur with i + 1.

This simulates the incrementing loop for (int i = 1; i <= N; i++).

Code (Java)

class Solution {
    public void printNos(int N) {
        printRecursively(1, N);
    }

    private void printRecursively(int current, int N) {
        if (current > N) return;
        System.out.print(current + " ");
        printRecursively(current + 1, N);
    }
}

Time and Space Complexity

• Time Complexity: O(N)

  • Each recursive call processes one number from 1 to N.

• Space Complexity: O(N) (due to recursion stack)

Dry Run

Input: N = 4

Function call trace:

• printRecursively(1, 4) → prints 1 → calls printRecursively(2, 4)

• printRecursively(2, 4) → prints 2 → calls printRecursively(3, 4)

• printRecursively(3, 4) → prints 3 → calls printRecursively(4, 4)

• printRecursively(4, 4) → prints 4 → calls printRecursively(5, 4)

• printRecursively(5, 4) → returns (base case)

Output: 1 2 3 4

Conclusion

This problem demonstrates how recursion can replace iteration for generating sequences. It's a common interview test to evaluate fundamental recursion skills and understanding of base and recursive cases.