Submission #399410

#	Time	Username	Problem	Language	Result	Execution time	Memory
399410		couplefire	Kangaroo (CEOI16_kangaroo)	C++17	100 / 100	22 ms	14076 KiB

kangaroo

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

const int N = 2000+5;
const int MOD = 1000000007;

inline int add(int a, int b){return (a+b>=MOD)?a+b-MOD:a+b;}
inline void inc(int& a, int b){a = add(a, b);}
inline int sub(int a, int b){return (a-b<0)?a-b+MOD:a-b;}
inline void dec(int &a, int b){a = sub(a, b);}
inline int mul(int a, int b){return 1ll*a*b%MOD;}
inline void grow(int &a, int b){a = mul(a, b);}

int n, cs, cf;
int dp[N][N];

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cin >> n >> cs >> cf;
    if(cs > cf) cs = n+1-cs, cf = n+1-cf;
    dp[0][0] = 1;
    for(int i = 1; i<cs; i++) for(int j = 1; j<=i; j++) inc(dp[i][j], add(mul(dp[i-1][j+1], j), mul(dp[i-1][j-1], j)));
    for(int j = 1; j<=cs; j++) inc(dp[cs][j], add(dp[cs-1][j], dp[cs-1][j-1]));
    for(int i = cs+1; i<cf; i++) for(int j = 1; j<=i; j++) inc(dp[i][j], add(mul(dp[i-1][j+1], j), mul(dp[i-1][j-1], j-1)));
    for(int j = 1; j<=cf; j++) inc(dp[cf][j], add(dp[cf-1][j], dp[cf-1][j-1]));
    for(int i = cf+1; i<=n; i++) for(int j = 1; j<=i; j++) inc(dp[i][j], add(mul(dp[i-1][j+1], j), mul(dp[i-1][j-1], j-2)));
    cout << dp[n][1] << endl;
}

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