← DSA Foundations
easyBasic Recursion

Sum of first N Natural Numbers

Recursively calculate the sum of the first n positive integers.

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Work through the cases

3 examples

Input notation

An integer n from 0 through 500.

Expected result

The sum.
Case 1

Input

6

Expected output

21
Case 2

Input

0

Expected output

0
Case 3

Input

1

Expected output

1

These notes use simplified input/output notation. The linked platform defines the method signature and judge format.

FROM IDEA TO IMPLEMENTATION

Understand the approach.

Compare the documented approaches below. Brute force, better, and optimal versions appear where available; a direct exercise may need only one approach.

Return n plus the sum through n-1. The base case is zero. Time and call-stack space: O(n).

import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        ArrayList<Long> values = new ArrayList<>();
        while (in.hasNextLong()) values.add(in.nextLong());
        long[] a = values.stream().mapToLong(Long::longValue).toArray();
        long n = a[0];
        class R { long f(long k){return k==0?0:k+f(k-1);} } System.out.println(new R().f(n));
    }
}

Reference code. Platform solutions may use supplied node types, imports, or collection libraries; foundational examples are complete programs.