Hdu 5225 Tom and permutation (backtracking)

Hdu 5225 Tom and permutation (backtracking)

 

 

#include 
 
  #include 
  
   #include using namespace std;typedef long long ll;const int maxn = 100;const int mod = 1e9+7;int N, ans, V[maxn + 5], A[maxn + 5];ll S[maxn + 5], L[maxn + 5];int query(int x) {int ret = 0;for (int i = 1; i < x; i++) {if (V[i] == 0)ret++;}return ret;}int dfs(int d, int s) {if (d > N)return 0;if (s == 0) {ans = (ans + S[N-d+1]) % mod;return L[N-d+1];} else {int ret = 0;for (int i = 1; i <= A[d]; i++) {if (V[i]) continue;V[i] = 1;int s = query(i);ll temp = dfs(d + 1, i == A[d] ? 1 : 0);ans = (ans + temp * s % mod) % mod;ret = (ret + temp) % mod;V[i] = 0;}return ret;}}int main () {S[1] = 0;L[1] = 1;for (int i = 2; i <= maxn; i++) {S[i] = S[i-1] * i % mod+ L[i-1] * ((1LL * i * (i-1) / 2) % mod) % mod;L[i] = L[i-1] * i % mod;}while (scanf(%d, &N) == 1) {ans = 0;memset(V, 0, sizeof(V));for (int i = 1; i <= N; i++)scanf(%d, &A[i]);dfs(1, 1);printf(%d, ans);}return 0;}