이 제출은 이전 버전의 oj.uz에서 채점하였습니다. 현재는 제출 당시와는 다른 서버에서 채점을 하기 때문에, 다시 제출하면 결과가 달라질 수도 있습니다.
#ifdef LOCAL
#define _GLIBCXX_DEBUG
#endif
#include <bits/stdc++.h>
#ifndef LOCAL
// #pragma GCC optimize("O3")
// #pragma GCC optimize("Ofast")
// #pragma GCC optimize("unroll-loops")
// #pragma GCC target("avx,avx2,bmi,bmi2,popcnt,lzcnt")
#endif
using namespace std;
using ll = long long;
using ld = long double;
#define x first
#define y second
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
mt19937_64 mt(time(0));
void solve();
void init();
int32_t main() {
#ifndef LOCAL
cin.tie(nullptr)->sync_with_stdio(false);
#endif
cout << fixed << setprecision(30);
init();
int t = 1;
// cin >> t;
while (t--) {
solve();
}
}
const int LG = 19, N = 1 << LG, A = 26, mod = 1000000007;
int to_up[N], to_down[N], dp[N][A];
int add(int a, int b) {
return a + b >= mod ? a + b - mod : a + b;
}
int sub(int a, int b) {
return a >= b ? a - b : a - b + mod;
}
int mul(int a, int b) {
return 1ll * a * b % mod;
}
int bin_pow(int a, int x) {
int res = 1;
while (x) {
if (x & 1) {
res = mul(res, a);
}
a = mul(a, a);
x >>= 1;
}
return res;
}
struct segment_tree {
vector<int> st;
int size;
segment_tree(int sz = N) {
size = sz;
st.resize(2 * size, -1);
}
void update(int l, int r, int v) {
l += size;
r += size;
while (l <= r) {
if (l & 1) {
st[l] = max(st[l], v);
l++;
}
if (~r & 1) {
st[r] = max(st[r], v);
r--;
}
l >>= 1;
r >>= 1;
}
}
int get(int i) {
i += size;
int res = -1;
while (i) {
res = max(st[i], res);
i >>= 1;
}
return res;
}
};
void init() {}
void solve() {
int n, m;
cin >> n >> m;
segment_tree stl, str;
for (int i = 0; i < m; i++) {
int a, b;
cin >> a >> b; a--; b--;
if (a < b) {
str.update(a + 1, b, a);
} else {
stl.update(b + 1, a, b);
}
}
for (int i = 0; i < n; i++) {
to_up[i] = stl.get(i);
to_down[i] = str.get(i);
}
for (int i = 0; i < A; i++) {
dp[0][i] = 1;
}
// for (int i = 0; i < n; i++) {
// cout << to_up[i] << ' ';
// }
// cout << '\n';
// for (int i = 0; i < n; i++) {
// cout << to_down[i] << ' ';
// }
// cout << '\n';
for (int i = 1; i < n; i++) {
for (int j = 0; j < A; j++) {
int up = 0, down = 0, plate = 0, anywhere = 0;
if (to_up[i] != -1) {
up = dp[to_up[i]][j];
}
if (to_down[i] != -1) {
down = dp[to_down[i]][j];
}
if (to_up[i] > to_down[i]) {
plate = down;
} else {
plate = up;
}
anywhere = sub(dp[i - 1][j], sub(add(up, down), plate));
// cout << i << ' ' << j << ' ' << plate << ' ' << up << ' ' << down << ' ' << anywhere << '\n';
up = sub(up, plate);
down = sub(down, plate);
for (int l = 0; l < A; l++) {
dp[i][l] = add(dp[i][l], anywhere);
if (l <= j) {
dp[i][l] = add(dp[i][l], down);
}
if (l >= j) {
dp[i][l] = add(dp[i][l], up);
}
if (l == j) {
dp[i][l] = add(dp[i][l], plate);
}
}
}
}
int ans = 0;
// for (int i = 0; i < n; i++) {
// for (int j = 0; j < A; j++) {
// cout << dp[i][j] << ' ';
// }
// cout << '\n';
// }
for (int i = 0; i < A; i++) {
ans = add(ans, dp[n - 1][i]);
}
cout << ans << '\n';
// cout << sub(bin_pow(A, n), ans) << '\n';
}
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