Write a recursive function that takes an integer n as a parameter and returns the following sum/subtraction of odd numbers 1 - 3 + 5 - 7 + 9....+/- n (if n is even, the last term is n-1)

oddsumsub.cpp

#include <iostream>
#include <cmath>
using namespace std;
 
// 1 - 3 + 5 - 7 + 9 ... +/- n
 
// recursive
int f1(int n) {
	if (n == 1) return 1;
	if (n % 2 == 0) n--;
	if ((n + 1) % 4 == 0) return f1(n-2) - n;
	/* else */            return f1(n-2) + n;
}
 
// iterative (dynamic programming)
int f2(int n) {
	int R[n+1];
	R[1] = R[2] = 1;
	for (int i = 3; i <= n; ++i) {
		int k = i;
		if (k % 2 == 0) k--;
		if ((k + 1) % 4 == 0) R[i] = R[k-2] - k;
		else                  R[i] = R[k-2] + k;
	}
	return R[n];
}
 
int main(){
    	for (int i = 1; i <= 10; ++i)
    		std::cout << f2(2*i-1) << " ";
}