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  1. #include <iostream>
  2. #include <vector>
  3.  
  4. using namespace std;
  5.  
  6. const int MOD = 1e9 + 7;
  7. const int MAXN = 500005;
  8.  
  9. long long fact[MAXN], inv[MAXN];
  10.  
  11. long long power(long long base, long long exp) {
  12.     long long res = 1;
  13.     base %= MOD;
  14.     while (exp > 0) {
  15.         if (exp % 2 == 1) res = (res * base) % MOD;
  16.         base = (base * base) % MOD;
  17.         exp /= 2;
  18.     }
  19.     return res;
  20. }
  21.  
  22. long long modInverse(long long n) {
  23.     return power(n, MOD - 2);
  24. }
  25.  
  26. void precompute() {
  27.     fact[0] = 1;
  28.     inv[0] = 1;
  29.     for (int i = 1; i < MAXN; i++) {
  30.         fact[i] = (fact[i - 1] * i) % MOD;
  31.     }
  32.     inv[MAXN - 1] = modInverse(fact[MAXN - 1]);
  33.     for (int i = MAXN - 2; i >= 1; i--) {
  34.         inv[i] = (inv[i + 1] * (i + 1)) % MOD;
  35.     }
  36. }
  37.  
  38. long long nCr(int n, int r) {
  39.     if (r < 0 || r > n) return 0;
  40.     return fact[n] * inv[r] % MOD * inv[n - r] % MOD;
  41. }
  42.  
  43. long long solve(int L, int R, const vector<long long>& a) {
  44.     if (L > R) return 1;
  45.  
  46.     int m = -1;
  47.     for (int i = 0; L + i <= R - i; ++i) {
  48.         if (a[L + i] == 1LL * (i + 1) * (R - (L + i) + 1)) {
  49.             m = L + i;
  50.             break;
  51.         }
  52.         if (a[R - i] == 1LL * (R - i - L + 1) * (i + 1)) {
  53.             m = R - i;
  54.             break;
  55.         }
  56.     }
  57.  
  58.     if (m == -1) return 0;
  59.  
  60.     long long left_ways = solve(L, m - 1, a);
  61.     if (left_ways == 0) return 0; 
  62.  
  63.     long long right_ways = solve(m + 1, R, a);
  64.     if (right_ways == 0) return 0; 
  65.  
  66.     long long ways = (left_ways * right_ways) % MOD;
  67.     ways = (ways * nCr(R - L, m - L)) % MOD;
  68.  
  69.     return ways;
  70. }
  71.  
  72. void run_case() {
  73.     int n;
  74.     cin >> n;
  75.     vector<long long> a(n + 1);
  76.     for (int i = 1; i <= n; i++) {
  77.         cin >> a[i];
  78.     }
  79.     cout << solve(1, n, a) << "\n";
  80. }
  81.  
  82. int main() {
  83.     ios_base::sync_with_stdio(false);
  84.     cin.tie(NULL);
  85.     precompute();
  86.  
  87.     int t;
  88.     cin >> t;
  89.     while (t--) {
  90.         run_case();
  91.     }
  92.  
  93.     return 0;
  94. }
Success #stdin #stdout 0.01s 11456KB
comments (?)
stdin 
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1 4 1
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1 6 1 2
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stdout 
2
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0
https://ideone.com/Aw0jtr
language:
C++14 (gcc 8.3)
created:
1 day ago
visibility:
public

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