이 제출은 이전 버전의 oj.uz에서 채점하였습니다. 현재는 제출 당시와는 다른 서버에서 채점을 하기 때문에, 다시 제출하면 결과가 달라질 수도 있습니다.
// Om Namah Shivaya
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
template<typename T> using Tree = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
typedef long long int ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define fastio ios_base::sync_with_stdio(false); cin.tie(NULL)
#define pb push_back
#define endl '\n'
#define sz(a) a.size()
#define setbits(x) __builtin_popcountll(x)
#define ff first
#define ss second
#define conts continue
#define ceil2(x, y) ((x + y - 1) / (y))
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define yes cout << "Yes" << endl
#define no cout << "No" << endl
#define rep(i, n) for(int i = 0; i < n; ++i)
#define rep1(i, n) for(int i = 1; i <= n; ++i)
#define rev(i, s, e) for(int i = s; i >= e; --i)
#define trav(i, a) for(auto &i : a)
template<typename T>
void amin(T &a, T b) {
a = min(a, b);
}
template<typename T>
void amax(T &a, T b) {
a = max(a, b);
}
#ifdef LOCAL
#include "debug.h"
#else
#define debug(x) 42
#endif
/*
*/
const int MOD = 1e9 + 7;
const int N = 5e5 + 5;
const int inf1 = int(1e9) + 5;
const ll inf2 = ll(1e18) + 5;
ll dp[N][26], pref[N][26];
void solve(int test_case)
{
ll n, m; cin >> n >> m;
vector<array<ll, 3>> a(m);
rep(i, m) {
ll l, r; cin >> l >> r;
ll t = 0;
if (l > r) {
swap(l, r);
t = 1;
}
a[i] = {t, l, r - 1};
}
rep(ch, 26) {
dp[1][ch] = 1;
pref[1][ch] = 1;
}
vector<ll> enter[n + 5][2], leave[n + 5][2];
rep(j, m) {
auto [t, l, r] = a[j];
enter[l + 1][t].pb(l);
leave[r + 2][t].pb(l);
}
multiset<ll> st1, st2;
st1.insert(0);
st2.insert(0);
for (int i = 2; i <= n; ++i) {
trav(l, enter[i][0]) {
st1.insert(l);
}
trav(l, enter[i][1]) {
st2.insert(l);
}
trav(l, leave[i][0]) {
st1.erase(st1.find(l));
}
trav(l, leave[i][1]) {
st2.erase(st2.find(l));
}
ll mx_les = *st1.rbegin();
ll mx_mor = *st2.rbegin();
/*
ll mx_les = 0, mx_mor = 0;
rep(j, m) {
auto [t, l, r] = a[j];
if (l <= i - 1 and r >= i - 1) {
if (t == 0) {
amax(mx_les, l);
}
else {
amax(mx_mor, l);
}
}
}
*/
rep(ch1, 26) {
auto &val = dp[i][ch1];
rep(ch2, 26) {
if (ch2 == ch1) {
// val += pref[i - 1][ch2];
}
else if (ch2 < ch1) {
val += pref[i - 1][ch2] - pref[mx_les][ch2];
}
else {
val += pref[i - 1][ch2] - pref[mx_mor][ch2];
}
}
val = (val % MOD + MOD) % MOD;
}
rep(ch, 26) {
pref[i][ch] = (pref[i - 1][ch] + dp[i][ch]) % MOD;
}
}
ll ans = 0;
rep(ch, 26) ans = (ans + pref[n][ch]) % MOD;
cout << ans << endl;
}
int main()
{
fastio;
int t = 1;
// cin >> t;
rep1(i, t) {
solve(i);
}
return 0;
}
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