This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include<iostream>
#include<cstring>
#include<vector>
#include<algorithm>
using namespace std;
int n,a[505],b[505],inv[505],mod=1e9+7;
vector <int> v,v1;
vector <vector<int> > v2;
vector <vector<vector<int> > > dp;
int sum(int a,int b) {
return (a+b)%mod;
}
int mul(int a,int b) {
return ((long long)a*b)%mod;
}
int exp(int a,int b) {
if(!b) return 1;
int ret=exp(a,b/2);
if(b%2==0) return mul(ret,ret);
return mul(mul(ret,ret),a);
}
int rec(int idx,int x,int cnt) {
if(idx>=v.size()-1) return 0;
if(x>=n) return 1;
if(dp[idx][x][cnt]!=-1) return dp[idx][x][cnt];
int ret=0;
ret=sum(ret,rec(idx,x+1,cnt)); // 0
//if(idx==0 && x==1 && cnt==1) cout << ret << "\n";
if(v[idx]>=a[x] && v[idx+1]-1<=b[x]) {
if(cnt<=v[idx+1]-1-v[idx]) ret=sum(ret,mul(rec(idx,x+1,cnt+1),mul(v[idx+1]-v[idx]-cnt,inv[cnt+1])));
}
for(int i=idx+1;i<v.size()-1;i++) {
if(v[i]>=a[x] && v[i+1]-1<=b[x]) ret=sum(ret,mul(rec(i,x+1,1),v[i+1]-v[i]));
}
dp[idx][x][cnt]=ret;
//cout << idx << " " << x << " " << cnt << " " << ret << "\n";
return ret;
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(0);
cin >> n;
for(int i=0;i<n;i++) {
cin >> a[i] >> b[i];
v.push_back(a[i]);
v.push_back(b[i]);
v.push_back(b[i]+1);
}
sort(v.begin(),v.end());
v.erase(unique(v.begin(),v.end()),v.end());
/*for(int i=0;i<v.size()-1;i++) {
cout << v[i] << " " << v[i+1]-1 << "\n";
}*/
/*for(int i=0;i<n;i++) {
a[i]=lower_bound(v.begin(),v.end(),a[i])-v.begin();
b[i]=lower_bound(v.begin(),v.end(),b[i])-v.begin();
}*/
for(int i=0;i<v.size()-1;i++) {
dp.push_back(v2);
for(int j=0;j<n;j++) {
dp[i].push_back(v1);
for(int k=0;k<=j;k++) {
dp[i][j].push_back(-1);
}
}
}
for(int i=1;i<=n;i++) {
inv[i]=exp(i,mod-2);
}
cout << rec(0,0,0)-1;
return 0;
}
Compilation message (stderr)
boat.cpp: In function 'int rec(int, int, int)':
boat.cpp:23:8: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
23 | if(idx>=v.size()-1) return 0;
| ~~~^~~~~~~~~~~~
boat.cpp:32:19: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
32 | for(int i=idx+1;i<v.size()-1;i++) {
| ~^~~~~~~~~~~
boat.cpp: In function 'int main()':
boat.cpp:59:15: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
59 | for(int i=0;i<v.size()-1;i++) {
| ~^~~~~~~~~~~
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