45 lines
1.1 KiB
C++
45 lines
1.1 KiB
C++
#include <cstdio>
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#include <cstring>
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#include <algorithm>
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#include <iostream>
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#include <string>
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#include <vector>
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#include <cmath>
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#include <queue>
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#include <map>
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#include <set>
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using namespace std;
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#define INF 1000000000
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#define N 111
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typedef __int64 LL;
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const LL mod = 1000000007;
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LL dp[N][N], C[N][N];
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int n, num[N];
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void init() {
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for(int i = 0; i < N; i++) {
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for(int j = 0; j <= i; j++) {
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if(j == 0 || j == i) C[i][j] = 1;
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else C[i][j] = (C[i-1][j-1] + C[i-1][j]) % mod;
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}
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}
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}
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LL dfs(int L, int R) {
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if(dp[L][R] != -1) return dp[L][R];
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if(L == R) return dp[L][R] = 1;
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dp[L][R] = (dfs(L+1, R) + dfs(L, R - 1)) % mod;
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for(int i = L+1; i <= R-1; i ++) {
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if(num[i-1] != num[i+1]) continue;
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dp[L][R] = (dp[L][R] + (dfs(L, i - 1) * dfs(i+1, R) % mod) * C[R- L][i - L]%mod) % mod;
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}
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return dp[L][R];
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}
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int main() {
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init();
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while(scanf("%d", &n) != EOF) {
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memset(dp, -1, sizeof(dp));
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for(int i = 1;i <= n; i++) scanf("%d", &num[i]);
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printf("%I64d\n", dfs(1, n));
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}
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return 0;
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}
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