C++ 递归算法求 1+2+3+1+2+3+...+N+N 的值

#include using namespace std;

int sum(int n) { if (n <= 0) { return 0; } return (n % 3) + sum(n-1) + n; }

int main() { int n; cout << "请输入N的值: "; cin >> n; cout << "1+2+3+1+2+3+...+" << n << "+" << n << " = " << sum(n) << endl; return 0; }