トリボナッチ数列（非再帰型）

Tribonacci.cpp

#include <iostream>
#include <sstream>
using namespace std;

// tribonacci sequence
long tribonacci(int n){
    long a=1, b=2, c=4;
    if(1>n){
        return 0;
    }else if(1==n){
        return a;
    }else if(2==n){
        return b;
    }else if(3==n){
        return c;
    }else{
        for(int i=n-3; i>0; i--){
            long tmp = a + b + c;
            a = b;
            b = c;
            c = tmp;
        }
        return c;
    }
}

int main(void){
    int n = 0;
    string str;
    getline(cin, str);
    istringstream iss(str);
    iss >> n;
    cout << n << endl;
    cout << tribonacci(n) << endl;
    return 0;
}