func_code_string: #include <bits/stdc++.h>   using namespace std;   int n, deg[51];   long long dp[2][51][51][51][51];   inline void add(long long &a, long long b) { a += b; }   void transition3(int b, int p1, int p2, int c1, int c2) {     long long from = dp[b][p1][p2][c1][c2] % 1000000007;     b ^= 1;     if (p1 > 0) add(dp[b][p1 - 1][p2][c1][c2 + 1], from * p1);     if (p2 > 0) add(dp[b][p1 + 1][p2 - 1][c1][c2 + 1], from * p2);     if (p1 > 0 && c1 > 0) add(dp[b][p1 - 1][p2][c1 - 1 + 1][c2], from * p1 * c1);     if (p2 > 0 && c1 > 0)       add(dp[b][p1 + 1][p2 - 1][c1 - 1 + 1][c2], from * p2 * c1);     if (p1 > 0 && c2 > 0)       add(dp[b][p1 - 1][p2][c1 + 1 + 1][c2 - 1], from * p1 * c2);     if (p2 > 0 && c2 > 0)       add(dp[b][p1 + 1][p2 - 1][c1 + 1 + 1][c2 - 1], from * p2 * c2);     if (p1 > 0 && c1 > 1)       add(dp[b][p1 - 1][p2][c1 - 2][c2], from * p1 * c1 * (c1 - 1) / 2);     if (p2 > 0 && c1 > 1)       add(dp[b][p1 + 1][p2 - 1][c1 - 2][c2], from * p2 * c1 * (c1 - 1) / 2);     if (p1 > 0 && c2 > 1)       add(dp[b][p1 - 1][p2][c1 + 2][c2 - 2], from * p1 * c2 * (c2 - 1) / 2);     if (p2 > 0 && c2 > 1)       add(dp[b][p1 + 1][p2 - 1][c1 + 2][c2 - 2], from * p2 * c2 * (c2 - 1) / 2);     if (p1 > 0 && c1 > 0 && c2 > 0)       add(dp[b][p1 - 1][p2][c1 - 1 + 1][c2 - 1], from * p1 * c1 * c2);     if (p2 > 0 && c1 > 0 && c2 > 0)       add(dp[b][p1 + 1][p2 - 1][c1 - 1 + 1][c2 - 1], from * p2 * c1 * c2);     if (p1 == 0 && p2 == 0 && c1 > 0) add(dp[b][c1 - 1][c2][0][1], from * c1);     if (p1 == 0 && p2 == 0 && c2 > 0) add(dp[b][c1 + 1][c2 - 1][0][1], from * c2);   }   void transition2(int b, int p1, int p2, int c1, int c2) {     long long from = dp[b][p1][p2][c1][c2] % 1000000007;     b ^= 1;     if (p1 > 0) add(dp[b][p1 - 1][p2][c1 + 1][c2], from * p1);     if (p2 > 0) add(dp[b][p1 + 1][p2 - 1][c1 + 1][c2], from * p2);     if (p1 > 0 && c1 > 0) add(dp[b][p1 - 1][p2][c1 - 1][c2], from * p1 * c1);     if (p2 > 0 && c1 > 0) add(dp[b][p1 + 1][p2 - 1][c1 - 1][c2], from * p2 * c1);     if (p1 > 0 && c2 > 0) add(dp[b][p1 - 1][p2][c1 + 1][c2 - 1], from * p1 * c2);     if (p2 > 0 && c2 > 0)       add(dp[b][p1 + 1][p2 - 1][c1 + 1][c2 - 1], from * p2 * c2);     if (p1 == 0 && p2 == 0 && c1 > 0) add(dp[b][c1 - 1][c2][1][0], from * c1);     if (p1 == 0 && p2 == 0 && c2 > 0) add(dp[b][c1 + 1][c2 - 1][1][0], from * c2);   }   void solveDP() {     int ip1 = 0, ip2 = 0, ic1 = 0, ic2 = 0;     if (deg[1] == 3 && deg[2] == 3) ip2 = 1, ic2 = 1;     if (deg[1] == 3 && deg[2] == 2) ip2 = 1, ic1 = 1;     if (deg[1] == 2 && deg[2] == 3) ip1 = 1, ic2 = 1;     if (deg[1] == 2 && deg[2] == 2) ip1 = 1, ic1 = 1;     dp[0][ip1][ip2][ic1][ic2] = 1;     for (int v = 2; v <= n - 1; ++v) {       int b = (v & 1);       for (int p1 = 0; p1 <= v; ++p1) {         for (int p2 = 0; p1 + p2 <= v; ++p2) {           for (int c1 = 0; p1 + p2 + c1 <= v; ++c1) {             for (int c2 = 0; p1 + p2 + c1 + c2 <= v; ++c2) {               dp[b ^ 1][p1][p2][c1][c2] = 0;             }           }         }       }       for (int p1 = 0; p1 <= v; ++p1) {         for (int p2 = 0; p1 + p2 <= v; ++p2) {           for (int c1 = 0; p1 + p2 + c1 <= v; ++c1) {             for (int c2 = 0; p1 + p2 + c1 + c2 <= v; ++c2) {               if (deg[v + 1] == 3)                 transition3(b, p1, p2, c1, c2);               else                 transition2(b, p1, p2, c1, c2);             }           }         }       }     }     cout << dp[(n & 1)][0][0][0][0] % 1000000007;   }   void solve() {     cin >> n;     for (int i = 1; i <= (n); ++i) cin >> deg[i];     solveDP();   }   int32_t main() {     ios_base::sync_with_stdio(0);     cin.tie(NULL);     solve();     return 0;   }   