이 제출은 이전 버전의 oj.uz에서 채점하였습니다. 현재는 제출 당시와는 다른 서버에서 채점을 하기 때문에, 다시 제출하면 결과가 달라질 수도 있습니다.
#include <bits/stdc++.h>
using namespace std;
#define rep(i,a,n) for (int i=a;i<n;i++)
#define per(i,a,n) for (int i=n-1;i>=a;i--)
#define pb push_back
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define SZ(x) ((int)(x).size())
typedef long long ll;
typedef long double ld;
typedef pair<ll, int> ii;
const int mod = (int) 1e9 + 7;
const ll inf = 1LL << 60;
const int maxn = (int) 1005;
const ld eps = 1e-9;
void add(int &a, ll b) {
a += (b % mod);
while (a >= mod) a -= mod;
}
ll pw(ll a, int b) {
ll res = 1;
while (b) {
if(b & 1) res = res * a % mod;
a = a * a % mod;
b >>= 1;
}
return res;
}
int n, m, a[maxn], b[maxn], sz[maxn];
int f[maxn][maxn], inv[maxn], C[maxn][maxn];
vector<int> p;
int main() {
//freopen("test.txt", "r", stdin);
ios_base::sync_with_stdio(0); cin.tie(0);
cin >> n;
rep(i, 1, n + 1) {
cin >> a[i] >> b[i];
b[i] ++;
p.pb(a[i]);
p.pb(b[i]);
inv[i] = pw(i, mod - 2);
}
p.pb(0);
sort(all(p));
p.resize(unique(all(p)) - p.begin());
rep(i, 1, n + 1) a[i] = lower_bound(all(p), a[i]) - p.begin();
rep(i, 1, n + 1) b[i] = lower_bound(all(p), b[i]) - p.begin();
m = SZ(p) - 2;
rep(i, 1, m + 1) sz[i] = p[i + 1] - p[i];
rep(i, 0, m + 1) f[0][i] = 1;
rep(i, 1, n + 1) {
rep(k, a[i], b[i]) {
ll cnt = 1, x = sz[k], y = 1;
per(j, 0, i) {
if(a[j + 1] <= k && k < b[j + 1]) {
cnt = ((cnt * x % mod) * inv[y]) % mod;
x ++, y ++;
}
add(f[i][k], cnt * f[j][k - 1]);
}
}
rep(j, 1, m + 1) add(f[i][j], f[i][j - 1]);
}
int ans = 0;
rep(i, 1, n + 1) add(ans, f[i][m]);
cout << ans << endl;
}
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