#include <bits/stdc++.h> using namespace std; #define endl '\n' #define st first #define nd second #define pb push_back #define sz(x) (int)(x).size() #define all(x) (x).begin(), (x).end() #define ll long long ll mod=1000000007; int inf=1000000007; ll infl=1000000000000000007; const int N=3007; ll DP[N][N][2]; int main() { ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0); ll n,m; cin>>n>>m; DP[1][1][1]=m; for(ll i=1;i<n;i++) { for(ll j=1;j<=min(n,m);j++) { DP[i+1][j+1][1]=(DP[i+1][j+1][1]+DP[i][j][0]*(m-j))%mod; DP[i+1][j][0]=(DP[i+1][j][0]+DP[i][j][0]*j)%mod; DP[i+1][j][0]=(DP[i+1][j][0]+DP[i][j][1]*j)%mod; DP[i+1][j][1]=(DP[i+1][j][1]+DP[i][j][1]*(m-j))%mod; } } ll ans=0; for(ll j=1;j<=min(n,m);j++) { ans=(ans+(DP[n-1][j][0]+DP[n-1][j][1])*j)%mod; } cout<<ans; return 0; }
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 | #include <bits/stdc++.h> using namespace std; #define endl '\n' #define st first #define nd second #define pb push_back #define sz(x) (int)(x).size() #define all(x) (x).begin(), (x).end() #define ll long long ll mod=1000000007; int inf=1000000007; ll infl=1000000000000000007; const int N=3007; ll DP[N][N][2]; int main() { ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0); ll n,m; cin>>n>>m; DP[1][1][1]=m; for(ll i=1;i<n;i++) { for(ll j=1;j<=min(n,m);j++) { DP[i+1][j+1][1]=(DP[i+1][j+1][1]+DP[i][j][0]*(m-j))%mod; DP[i+1][j][0]=(DP[i+1][j][0]+DP[i][j][0]*j)%mod; DP[i+1][j][0]=(DP[i+1][j][0]+DP[i][j][1]*j)%mod; DP[i+1][j][1]=(DP[i+1][j][1]+DP[i][j][1]*(m-j))%mod; } } ll ans=0; for(ll j=1;j<=min(n,m);j++) { ans=(ans+(DP[n-1][j][0]+DP[n-1][j][1])*j)%mod; } cout<<ans; return 0; } |