Submission #671683

#	Time	Username	Problem	Language	Result	Execution time	Memory
671683		uylulu	Kangaroo (CEOI16_kangaroo)	C++14	100 / 100	173 ms	125940 KiB

kangaroo

#include <bits/stdc++.h>
using namespace std;

#define ld long double
#define int long long
#define endl "\n"

const int N = 2000,mod = 1e9 + 7;

int dp[N + 1][N + 1][2][2];
int n,a,b;

int f(int curr,int comp,bool front,bool back) {
    if(curr == n + 1) {
        return (comp == 1);
    }
    if(dp[curr][comp][front][back] != -1) return dp[curr][comp][front][back];

    if(curr == a) {
        int res = 0;
        if(!front) {
            res = (res + f(curr + 1,comp,1,back))%mod;
            res = (res + f(curr + 1,comp + 1,0,back))%mod;
        }
        return dp[curr][comp][front][back] = res;
    }
    if(curr == b) {
        int res = 0;
        if(!back) {
            res = (res + f(curr + 1,comp + 1,front,0))%mod;
            res = (res + f(curr + 1,comp,front,1))%mod;
        }
        return dp[curr][comp][front][back] = res;
    }
    int res = 0;
    if(comp > 1) {
        res = (res + (f(curr + 1,comp - 1,front,back) * (comp - 1))%mod)%mod;
    }
    res = (res + (f(curr + 1,comp + 1,front,back) * (comp - 1))%mod)%mod;

    if(curr < a) {
        if(!front) {
            res = (res + f(curr + 1,comp + 1,0,back))%mod;
            res = (res + f(curr + 1,comp,1,back))%mod;
        }
    }
    if(curr < b) {
        if(!back) {
            res = (res + f(curr + 1,comp + 1,front,0))%mod;
            res = (res + f(curr + 1,comp,front,1))%mod;
        }
    }
    return dp[curr][comp][front][back] = res;

}

signed main() {
    ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
    // freopen("in.txt","r",stdin);
    // freopen("out.txt","w",stdout);

    cin>>n>>a>>b;

    memset(dp,-1,sizeof(dp));

    cout<<f(2,1,0,0)<<endl;

    vector<int> asd;
    for(int i = 1;i <= n;i++) {
        asd.push_back(i);
    }

    return 0;
}       

• Subtask 1: 6 / 6
• Subtask 2: 30 / 30
• Subtask 3: 15 / 15
• Subtask 4: 49 / 49