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General

# Author Problem Lang Verdict Time Memory Sent Judged
86746688 Practice:
JKLover
995F - 17 GNU C++17 Accepted 327 ms 39508 KB 2020-07-13 15:25:29 2020-07-13 15:25:29

→ Source
//%std
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
inline int read()
{
int out = 0, fh = 1;
char jp = getchar();
while ((jp > '9' || jp < '0') && jp != '-')
jp = getchar();
if (jp == '-')
fh = -1, jp = getchar();
while (jp >= '0' && jp <= '9')
out = out * 10 + jp - '0', jp = getchar();
return out * fh;
}
void print(int x)
{
if (x >= 10)
print(x / 10);
putchar('0' + x % 10);
}
void write(int x, char c)
{
if (x < 0)
putchar('-'), x = -x;
print(x);
putchar(c);
}
const int P = 1e9 + 7;
int add(int a, int b)
{
return a + b >= P ? a + b - P : a + b;
}
void inc(int &a, int b)
{
a = add(a, b);
}
int mul(int a, int b)
{
return 1LL * a * b % P;
}
const int N = 3e3 + 10;
int n, D, inv[N];
int dp[N][N]; // dp[x][i] := subtree x, choose from [1, i]
vector<int> G[N];
void dfs(int x)
{
for (int y : G[x])
dfs(y);
for (int i = 1; i <= n + 1; ++i)
{
int prod = 1;
for (int y : G[x])
prod = mul(prod, dp[y][i]);
dp[x][i] = add(dp[x][i - 1], prod);
}
}
int main()
{
n = read(), D = read();
inv[1] = 1;
for (int i = 2; i <= n; ++i)
{
inv[i] = mul(P - P / i, inv[P % i]);
G[read()].push_back(i);
}
dfs(1);
int ans = 0;
for (int i = 1; i <= n + 1; ++i)
{
int tmp = dp[1][i];
for (int j = 1; j <= n + 1; ++j)
if (i != j)
{
tmp = mul(tmp, add(P, D - j));
if (i > j)
tmp = mul(tmp, inv[i - j]);
else
tmp = mul(tmp, P - inv[j - i]);
}
inc(ans, tmp);
}
cout << ans << '\n';
return 0;
}


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