Ex_01.java

package Big_O;

public class Ex_01 {
	public static int sum(int n) {
		if (n <= 0) {
			return 0;
		}
		return n + sum(n-1);
	}
	
	public static void main(String[] args) {
		int s = sum(4);
		System.out.println(s);
	}
}