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 <bits/stdc++.h>
using namespace std;
const int nx=5e5+5, kx=26, mod=1e9+7;
int n, m, a, b, dp[nx][kx], res, up[nx], dn[nx], qsup[nx], qsdn[nx];
stack<int> cu, cd;
void calcup(int idx, int mul)
{
int tmp=0;
for (int i=25; i>=0; i--)
{
tmp=((tmp+dp[idx][i]*mul)%mod+mod)%mod;
qsup[i]=(qsup[i]+tmp)%mod;
}
}
void calcdn(int idx, int mul)
{
//cout<<"calcdn "<<idx<<' '<<mul<<'\n';
int tmp=0;
for (int i=0; i<26; i++)
{
tmp=((tmp+dp[idx][i]*mul)%mod+mod)%mod;
qsdn[i]=(qsdn[i]+tmp)%mod;
}
}
int main()
{
cin.tie(NULL)->sync_with_stdio(false);
cin>>n>>m;
for (int i=1; i<=m; i++)
{
cin>>a>>b;
if (a<b) dn[a+1]=max(dn[a+1], b);
else up[b+1]=max(up[b+1], a);
}
for (int i=n; i>=1; i--)
{
for (int c=0; c<26; c++)
{
dp[i][c]=1;
if (c>0) dp[i][c]=(dp[i][c]+qsdn[c-1])%mod;
if (c<25) dp[i][c]=(dp[i][c]+qsup[c+1])%mod;
//cout<<"debug "<<i<<' '<<c<<' '<<dp[i][c]<<'\n';
}
cu.push(i);
calcup(i, 1);
cd.push(i);
calcdn(i, 1);
while (!cd.empty()&&cd.top()<=up[i]) calcdn(cd.top(), -1), cd.pop();
while (!cu.empty()&&cu.top()<=dn[i]) calcup(cu.top(), -1), cu.pop();
}
for (int c=0; c<26; c++) res=(res+dp[1][c])%mod;
cout<<res;
}
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