【BZOJ4723】【POI2017】Sabota?

#include<bits/stdc++.h>
using namespace std;
const int MAXN = 500005;
template <typename T> void chkmax(T &x, T y) {x = max(x, y); }
template <typename T> void chkmin(T &x, T y) {x = min(x, y); }
template <typename T> void read(T &x) {
x = 0; int f = 1;
char c = getchar();
for (; !isdigit(c); c = getchar()) if (c == '-') f = -f;
for (; isdigit(c); c = getchar()) x = x * 10 + c - '0';
x *= f;
}
template <typename T> void write(T x) {
if (x < 0) x = -x, putchar('-');
if (x > 9) write(x / 10);
putchar(x % 10 + '0');
}
template <typename T> void writeln(T x) {
write(x);
puts("");
}
double ans, dp[MAXN];
int n, m, size[MAXN];
vector <int> a[MAXN];
void work(int pos) {
size[pos] = 1;
for (unsigned i = 0; i < a[pos].size(); i++) {
work(a[pos][i]);
size[pos] += size[a[pos][i]];
}
if (size[pos] == 1) {
dp[pos] = 1;
return;
}
for (unsigned i = 0; i < a[pos].size(); i++)
chkmax(dp[pos], min(dp[a[pos][i]], size[a[pos][i]] / (size[pos] - 1.0)));
if (size[pos] > m) chkmax(ans, dp[pos]);
}
int main() {
read(n), read(m);
for (int i = 2; i <= n; i++) {
int x; read(x);
a[x].push_back(i);
}
ans = 0;
work(1);
printf("%.10lf\n", ans);
return 0;
}