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 int long long
const int maxn = 5e5 + 5, M = 1e9 + 7;
int n, m;
int a[maxn][26];
int fen[maxn][26][2];
void update(int id, int j, int t, int val) {
if (t==2) {
a[id][j] += val;
update(id, j, 0, val); update(id, j, 1, val);
return;
}
while (id<=n+1) {
fen[id][j][t] = fen[id][j][t] + val + M;
while (val>M) val -= M;
id += (id & -id);
}
}
int qry(int id, int j, int t) {
int sum = 0;
while (id>=1) {
sum = sum + fen[id][j][t];
while (sum>M) sum -= M;
id -= (id & -id);
}
return sum;
}
signed main() {
ios::sync_with_stdio(0); cin.tie(0);
cin >> n >> m;
int R[n+1][2];
for (int i=1;i<=n;i++) R[i][0] = -1, R[i][1] = -1;
for (int i=1;i<=m;i++) {
int x, y;
cin >> x >> y;
if (x<y) R[x][0] = max(y, R[x][0]);
if (x>y) R[y][1] = max(x, R[y][1]);
}
set<int> exist[2];
for (int i=1;i<=n+1;i++) exist[0].insert(i), exist[1].insert(i);
for (int i=0;i<26;i++) update(n, i, 2, 1);
for (int l=n-1;l>=1;l--) {
for (int t=0;t<=1;t++) {
int r = R[l][t];
if (r==-1) continue;
while (true) {
int cur = *exist[!t].lower_bound(l+1);
if (cur > r) break;
for (int j=0;j<26;j++) update(cur, j, !t, -a[cur][j]);
exist[!t].erase(cur);
// cout << "del " << cur << " " << !t << endl;
}
}
int cur = 1;
for (int j=0;j<26;j++) cur = (cur + qry(n, j, 1) - qry(l, j, 1)) % M;
for (int j=0;j<26;j++) {
cur = (cur - (qry(n, j, 1) - qry(l, j, 1)) + M) % M;
update(l, j, 2, cur);
cur = (cur + (qry(n, j, 0) - qry(l, j, 0)) + M) % M;
}
}
int ans = 0;
for (int i=0;i<26;i++) ans = (ans + a[1][i]) % M;
cout << ans << endl;
}
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