#pragma GCC target ("avx2")
#pragma GCC optimization ("O3")
#pragma GCC optimization ("unroll-loops")
#include <bits/stdc++.h>
typedef long long ll;
using namespace std;
const ll MOD = 1e9 + 7;
ll modpow(ll n, ll e) {
if(e == 0) return 1;
ll x = modpow(n, e/2);
x *= x; x %= MOD;
if(e%2) {
x *= n;
x %= MOD;
}
return x;
}
ll modinv(ll n) {
return modpow(n, MOD - 2);
}
ll fact[5001];
ll invfact[5001];
ll binom(ll n, ll k) { // Assumes k is quite small
if(k > n) return 0;
if(k == n) return 1;
ll ans = 1;
if(n-k <= 5000) {
ans = fact[n];
ans *= invfact[n-k];
ans %= MOD;
}
else {
for(ll i = n-k+1; i <= n; i++) {
ans *= i;
ans %= MOD;
}
}
ans *= invfact[k];
ans %= MOD;
return ans;
}
void precalc() {
fact[0] = 1;
for(int i = 1; i <= 5000; i++) {
fact[i] = (fact[i-1] * i) % MOD;
}
for(int i = 0; i <= 5000; i++) {
invfact[i] = modinv(fact[i]);
}
}
ll ans[501][1001], pref[501][1001];
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
precalc();
int n;
cin >> n;
vector<int> nums(2 * n), a(n+1), b(n+1);
for(int i = 1; i <= n; i++) {
cin >> a[i] >> b[i];
b[i]++;
nums[2*i - 2] = a[i];
nums[2*i - 1] = b[i];
}
sort(nums.begin(), nums.end());
nums.resize(unique(nums.begin(), nums.end()) - nums.begin());
for(int i = 1; i <= n; i++) {
for(int j = 1; j < nums.size(); j++) {
if(!(a[i] <= nums[j-1] && nums[j] <= b[i])) {
continue;
}
ll siz = nums[j] - nums[j-1];
ans[i][j] += siz * (pref[i-1][j-1] + 1)
ans[i][j] %= MOD;
int g = 1;
for(int k = i-1; k > 0; k--) {
if(a[k] <= nums[j-1] && nums[j] <= b[k]) {
g++;
ll x = siz;
for(int l = 2; l <= g; l++) {
x *= siz-l+1;
x %= MOD;
x *= modinv(l);
x %= MOD;
ans[i][j] += ((x * binom(g-2, l-2)) % MOD) * (1 + pref[k-1][j-1]);
ans[i][j] %= MOD;
}
}
}
}
for(int j = 1; j < nums.size(); j++) {
pref[i][j] += ans[i][j] + pref[i][j-1] + pref[i-1][j] - pref[i-1][j-1];
pref[i][j] += MOD;
pref[i][j] %= MOD;
}
}
cout << pref[n][nums.size()-1] << endl;
}
Compilation message
boat.cpp:2: warning: ignoring '#pragma GCC optimization' [-Wunknown-pragmas]
2 | #pragma GCC optimization ("O3")
|
boat.cpp:3: warning: ignoring '#pragma GCC optimization' [-Wunknown-pragmas]
3 | #pragma GCC optimization ("unroll-loops")
|
boat.cpp: In function 'int main()':
boat.cpp:78:26: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
78 | for(int j = 1; j < nums.size(); j++) {
| ~~^~~~~~~~~~~~~
boat.cpp:83:52: error: expected ';' before 'ans'
83 | ans[i][j] += siz * (pref[i-1][j-1] + 1)
| ^
| ;
84 | ans[i][j] %= MOD;
| ~~~
boat.cpp:101:26: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
101 | for(int j = 1; j < nums.size(); j++) {
| ~~^~~~~~~~~~~~~
