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;
#define ll long long
#define ar array
const int mxN=5e5, M=1e9+7, INF=69696969;
int n, m, first[mxN][2], dp[mxN][2][26], pbad[mxN+1][2], base[mxN][26];
vector<int> bad[mxN][2];
struct ST {
int st[1<<20], lz[1<<20];
void psh(int u, int l, int r) {
if (lz[u]) {
st[u]+=lz[u];
if (l!=r)
lz[2*u]+=lz[u], lz[2*u+1]+=lz[u];
lz[u]=0;
}
}
void upd(int ql, int qr, int x, int u=1, int l=0, int r=n-1) {
psh(u, l, r);
if (l>qr||r<ql)
return;
if (ql<=l&&r<=qr) {
lz[u]=x;
psh(u, l, r);
return;
}
int mid=(l+r)/2;
upd(ql, qr, x, 2*u, l, mid);
upd(ql, qr, x, 2*u+1, mid+1, r);
st[u]=min(st[2*u], st[2*u+1]);
}
int qry(int u=1, int l=0, int r=n-1) {
psh(u, l, r);
if (st[u]>0)
return -1;
if (l==r)
return l;
int mid=(l+r)/2;
int ls=qry(2*u, l, mid);
return ls!=-1?ls:qry(2*u+1, mid+1, r);
}
} st[2];
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
cin >> n >> m;
for (int i=0; i<m; ++i) {
int a, b;
cin >> a >> b, --a, --b;
if (a<b) {
++a;
bad[a][0].push_back(b);
st[0].upd(a, b, 1);
++pbad[a][0], --pbad[b+1][0];
} else {
swap(a, b);
++a;
bad[a][1].push_back(b);
st[1].upd(a, b, 1);
++pbad[a][1], --pbad[b+1][1];
}
}
for (int i=1; i<n; ++i) {
for (int j : {0, 1})
pbad[i][j]+=pbad[i-1][j];
for (int j=0; j<26; ++j)
base[i][j]=(!pbad[i][0]?j:0)+(!pbad[i][1]?25-j:0);
}
memset(first, -1, sizeof(first));
ll ans=26;
for (int i=0; i<n; ++i) {
//ar<ll, 26> cur;
//cur.fill(1);
ar<ll, 26> cur;
for (int j=0; j<26; ++j)
cur[j]=base[i][j];
for (int j : {0, 1})
if (first[i][j]!=-1)
for (int k=0; k<26; ++k)
cur[k]=(cur[k]+dp[i][j][k]-dp[first[i][j]][j][k]+M)%M;
for (int j : {0, 1}) {
for (int k : bad[i][j])
st[j].upd(i, k, -1);
for (int pos=st[j].qry(); pos!=-1; pos=st[j].qry()) {
st[j].upd(pos, pos, INF);
first[pos][j]=i;
}
}
//if (i==n-1)
for (int j=0; j<26; ++j)
ans=(ans+cur[j])%M;
int ic=0;
for (int j=0; j<26; ++j) {
dp[i+1][0][j]=(dp[i][0][j]+ic)%M;
ic=(ic+cur[j])%M;
}
int dc=0;
for (int j=25; ~j; --j) {
dp[i+1][1][j]=(dp[i][1][j]+dc)%M;
dc=(dc+cur[j])%M;
}
}
cout << ans;
return 0;
}
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