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OI-codes/S2OJ/1541/1541.cpp

72 lines
1.2 KiB
C++

#include <iostream>
#include <vector>
using std::cin;
using std::cout;
const char endl = '\n';
const int N = 305,
M = 1e5 + 5;
const int mod = 1e9 + 7;
int n, m, b[N], fa[N];
long long f[N][M];
std::vector<int> g[N];
bool vis[N];
void dfs(int u) {
std::fill_n(f[u] + 1, m + 1, 1);
for (const int& v : g[u]) {
dfs(v);
for (int j = 1; j <= m + 1; j++) {
f[u][j] = (f[u][j] * f[v][j - vis[v]]) % mod;
}
}
if (u) {
for (int i = 1; i <= m + 1; i++) {
f[u][i] = (f[u][i] + f[u][i - 1]) % mod;
}
}
}
int main() {
std::ios::sync_with_stdio(false);
cin.tie(nullptr);
cin >> n >> m;
for (int i = 1; i <= n; i++) {
cin >> b[i];
}
for (int i = 1; i <= n; i++) {
vis[i] = true;
if (b[i] == -1) {
fa[i] = 0;
} else {
fa[i] = b[i];
for (int j = b[i] + 1; j < i; j++) {
if (fa[j] <= b[i]) {
fa[j] = i;
vis[j] = false;
}
}
}
}
for (int i = 1; i <= n; i++) {
g[fa[i]].emplace_back(i);
}
dfs(0);
cout << f[0][m + 1] << endl;
return 0;
}