题解 | 小红蹦跳蹦跳

#include<bits/stdc++.h>
using namespace std;
#define ll long long
const ll N = 1e9+7;
void solve()
{
    int n;
    cin>>n;
    int jzj = 0;
    int jzo = 0;

    int ozj = 0;
    int ozo = 0;
    vector<vector<ll>>dp(n+1,vector<ll>(2));
    if(n == 1)
    {
        cout<<1<<'\n';
        return;
    }
    jzj+=1;
    dp[1][1] = 1;
    for(int i = 2;i<=n;++i)
    {
        if(i%2 == 1)
        {
            dp[i][1] = 1;
            dp[i][1] = (dp[i][1]+ozo)%N;
            dp[i][0] = (dp[i][0]+jzj)%N;
            jzj = (jzj+dp[i][1])%N;
            jzo = (jzo+dp[i][0])%N;

        }else if(i%2 == 0)
        {
            dp[i][0] = 1;
            dp[i][1] = (dp[i][1]+jzo)%N;
            dp[i][0] = (dp[i][0]+ozj)%N;
            ozj = (ozj+dp[i][1])%N;
            ozo = (ozo+dp[i][0])%N;;
        }
        //cout<<dp[i][0]+dp[i][1]<<" ";
    }
    ll ans = (dp[n][0]+dp[n][1])%N;
    cout<<ans<<'\n';
}
int main()
{
    int t = 1;
    //cin>>t;
    while(t--)
    {
        solve();
    }
}