#include <bits/stdc++.h> using namespace std; using ll = long long; #define thuhien "" #define re exit(0); const int mod = 1e9 + 7; const int maxn = 509; int pw(int a,int b) { if (b == 0) return 1; if (b % 2 == 0) { int x = pw(a,b / 2); return 1ll * x * x % mod; } else { int x = pw(a,b - 1); return 1ll * x * a % mod; } } int power[maxn]; int n,a[maxn],b[maxn]; pair <int,int> seg[2 * maxn]; vector <int> X; //int dp[maxn][2 * maxn][maxn],pref[maxn][2 * maxn]; //dp i j k:xet den vi tri i, nhom to nhat la nhom j va trong nhom nay dang co k phan tu int dp[2 * maxn][maxn],pref[2 * maxn],newpref[2 * maxn]; int main() { ios_base::sync_with_stdio(0); cin.tie(nullptr); if (fopen(thuhien".inp","r")) { freopen(thuhien".inp","r",stdin); freopen(thuhien".out","w",stdout); } cin >> n; for (int i = 1;i <= n;i++) { cin >> a[i] >> b[i]; X.push_back(a[i] - 1); X.push_back(b[i]); } sort(X.begin(),X.end()); X.resize(unique(X.begin(),X.end()) - X.begin()); for (int i = 0;i < (int)X.size() - 1;i++) { seg[i + 1] = {X[i],X[i + 1]}; } // for (int i = 1;i < 2 * n;i++) cout << seg[i].first << " " << seg[i].second << '\n'; //prebuild power for (int i = 1;i <= n;i++) power[i] = pw(i,mod - 2); //dynamic programming // cout << "POS GROUP NUM VALUE\n"; for (int i = 1;i <= n;i++) { for (int j = 1;j < 2 * n;j++) { newpref[j] = newpref[j - 1]; int len = seg[j].second - seg[j].first; for (int k = min(i,len);k >= 1;k--) { //khong chon // dp[i][j][k] = dp[i - 1][j][k]; //chon if (a[i] <= seg[j].first + 1 && seg[j].second <= b[i]) { //thang truoc do la thang duoi hoac khong thang nao if (k == 1) { dp[j][k] = (dp[j][k] + 1ll * (pref[j - 1] + 1) * len) % mod; } //thang truoc do la thang cung khoi dp[j][k] = (dp[j][k] + 1ll * dp[j][k - 1] * (len - k + 1) % mod * power[k]) % mod; } newpref[j] = (newpref[j] + dp[j][k]) % mod; } } for (int j = 1;j < 2 * n;j++) pref[j] = newpref[j]; } cout << pref[2 * n - 1]; }

boat.cpp: In function 'int main()':
boat.cpp:39:19: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   39 |            freopen(thuhien".inp","r",stdin);
      |            ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
boat.cpp:40:19: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   40 |            freopen(thuhien".out","w",stdout);
      |            ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~