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<vector>
#include<algorithm>
using namespace std;
using ll = long long;
#define forn(i, n) for(int i=0; i<(int)n; ++i)
#define dforn(i, n) for(int i=n-1; i>=0; --i)
#define PB push_back
const int MAXN=510, MOD=1000000007;
int n, le[MAXN], ri[MAXN], arr[2*MAXN][MAXN], inv[MAXN];
bool adj[2*MAXN][MAXN];
vector<int> pt;
ll ex(ll b, int e){
ll ret=1;
while(e>0){
if(e&1) ret=(ret*b)%MOD;
b = (b*b)%MOD;
e>>=1;
}
return ret;
}
int main(){
scanf("%d", &n);
forn(i, n+2) inv[i]=ex(i, MOD-2);
forn(i, n) scanf("%d %d", le+i, ri+i), pt.PB(le[i]), pt.PB(ri[i]+1);
sort(pt.begin(), pt.end());
pt.erase(unique(pt.begin(), pt.end()), pt.end());
forn(i, n){
auto itr = lower_bound(pt.begin(), pt.end(), le[i]);
for(; itr!=pt.end() && (*itr)<=ri[i]; ++itr) adj[itr-pt.begin()][i]=true;
}
forn(i, pt.size()-1){
forn(j, n){
arr[i+1][j]=(arr[i][j]-(j==0? 0 : arr[i][j-1]) + MOD)%MOD;
if(adj[i][j]){
int cnt=0, m=pt[i+1]-pt[i];
ll comb=m;
dforn(k, j){
int dif = (arr[i][k] - (k==0? 0 : arr[i][k-1]) + MOD)%MOD;
arr[i+1][j]=(arr[i+1][j] + comb*dif)%MOD;
if(adj[i][k]){
++cnt;
comb=(((comb*(m+cnt))%MOD)*((ll)inv[cnt+1]))%MOD;
}
}
arr[i+1][j]=(arr[i+1][j]+comb)%MOD;
}
}
forn(j, n-1) arr[i+1][j+1]=(arr[i+1][j+1]+arr[i+1][j])%MOD;
}
printf("%d\n", arr[pt.size()-1][n-1]);
}
Compilation message (stderr)
boat.cpp: In function 'int main()':
boat.cpp:26:10: warning: ignoring return value of 'int scanf(const char*, ...)' declared with attribute 'warn_unused_result' [-Wunused-result]
26 | scanf("%d", &n);
| ~~~~~^~~~~~~~~~
boat.cpp:28:21: warning: ignoring return value of 'int scanf(const char*, ...)' declared with attribute 'warn_unused_result' [-Wunused-result]
28 | forn(i, n) scanf("%d %d", le+i, ri+i), pt.PB(le[i]), pt.PB(ri[i]+1);
| ~~~~~^~~~~~~~~~~~~~~~~~~~~
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