用数组 递推 一样的 规律就是第n项=2*第（n-1）项+第（n-2）项

#include <bits/stdc++.h> using namespace std; int main(){ int n; cin >> n; int two = 0,three = 1; for(int i = 1;i < n;i++){ int th = three; three = two + three; two = two + th * 2; } cout << two * 2 + three * 3; return 0; }