#include <bits/stdc++.h> using namespace std; #define mod 1000000007 vector <int> ciag; int n, m; int el; long long res = 0; bool Check() { vector <bool> wyk(el + 1); bool xd = true; for (int i = 0; i < n - 1; i++) { if (wyk[ciag[i]]) xd = true; else if (xd) { wyk[ciag[i]] = true; xd = false; } } if (wyk[ciag[n - 1]]) { return true; } else return false; } void BackTrack(int maxx) { if ((int)ciag.size() == n) { if (Check()) { long long temp = 1; for (int i = m; i > m - maxx; i--) { temp *= i; temp %= mod; } res += temp; res %= mod; } return; } for (int i = 1; i <= min(maxx + 1, el); i++) { ciag.emplace_back(i); BackTrack(max(maxx, i)); ciag.pop_back(); } } int main() { ios_base::sync_with_stdio(false); cin.tie(NULL); cin >> n >> m; el = min(n, m); BackTrack(0); cout << res << "\n"; }
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 | #include <bits/stdc++.h> using namespace std; #define mod 1000000007 vector <int> ciag; int n, m; int el; long long res = 0; bool Check() { vector <bool> wyk(el + 1); bool xd = true; for (int i = 0; i < n - 1; i++) { if (wyk[ciag[i]]) xd = true; else if (xd) { wyk[ciag[i]] = true; xd = false; } } if (wyk[ciag[n - 1]]) { return true; } else return false; } void BackTrack(int maxx) { if ((int)ciag.size() == n) { if (Check()) { long long temp = 1; for (int i = m; i > m - maxx; i--) { temp *= i; temp %= mod; } res += temp; res %= mod; } return; } for (int i = 1; i <= min(maxx + 1, el); i++) { ciag.emplace_back(i); BackTrack(max(maxx, i)); ciag.pop_back(); } } int main() { ios_base::sync_with_stdio(false); cin.tie(NULL); cin >> n >> m; el = min(n, m); BackTrack(0); cout << res << "\n"; } |