I solved this a bit quickly, and I could update my highest rating.

## problem

$N$頂点で出次数が全て$1$であるような有向グラフが与えられる。 頂点の部分集合$M$で、以下の一連の操作をした後に頂点$0,1$が辺の向きを無視して連結になるような$M$の数を数えよ。

1. 頂点$n$を追加する
2. $v \in M$に関して、それから出る辺の行き先を頂点$n$にする

## solution

Think the directed chain from $0,1$ and the intersection. $O(N)$.

You can notice that: vertices which are not connected to both vertices $0,1$, are trivial. Also, vertices which is not able to be reached from both $0,1$ with the directed edges, can be ignored. You should consider only the two chains, one from the vertex $0$ and one from the vertex $1$.

Let the length of the two chains be $l_0, l_1$, and the number of vertices which are in the both chains be $c$. Then the answer is now $\rm{ans} = f(l_0, l_1, c) \cdot 2^{n - l_0 - l_1 + c}$.

To calculate the $f(l_0, l_1, c)$, do case analysis.

When two chains become equivalent, this becomes trivial: $f(l_0, l_1, c) = 2^{l_0 + l_1 - c} = 2^c$. When there are no intersections, there must be some vertices of $M$ in both chains. Therefore, $f(l_0, l_1, 0) = (2^{l_0} - 1) \cdot (2^{l_1} - 1)$. When one chain includes another chain properly: if there are some vertices of $M$ in the non-intersected part of including chain, there must be some vertices of $M$ in the incensed one. The constraint is only this, and $f(l_0, l_1, c) = (2^{l_0 - c} - 1) \cdot 2^{l_1} + 1 \cdot 2^{l_1}$ (when the chain $0$ includes the chain $1$, $l_1 = c$). The other cases, the chains are splitted into three parts. If the central part has vertices of $M$, other vertices has no constraints. Else, the central part has no vertices of $M$, the chains both has no vertices of $M$ or both has vertices of $M$. $f(l_0, l_1, c) = (2^c - 1) \cdot 2^{l_0 - c} \cdot 2^{l_1 - c} + 1 \cdot (1 \cdot 1 + (2^{l_0 - c} - 1) \cdot (2^{l_1 - c} - 1))$. ## implementation

#include <bits/stdc++.h>
#define repeat(i,n) for (int i = 0; (i) < (n); ++(i))
typedef long long ll;
using namespace std;
class Sunnygraphs { public: ll count(vector<int> const & a); };

vector<int> connected_vertices(vector<int> const & g, int root) {
vector<int> acc;
for (int v = root; find(acc.begin(), acc.end(), v) == acc.end(); v = g[v]) {
acc.push_back(v);
}
return acc;
}
ll Sunnygraphs::count(vector<int> const & a) {
int n = a.size();
vector<int> zeros = connected_vertices(a, 0);
vector<int> ones  = connected_vertices(a, 1);
vector<int> common;
repeat (i,n) {
if (find(zeros.begin(), zeros.end(), i) != zeros.end()
and find(ones.begin(), ones.end(), i) != ones.end()) {
common.push_back(i);
}
}
int z =  zeros.size();
int o =   ones.size();
int c = common.size();
if (common.empty()) {
return ((1ll << z) - 1) * ((1ll << o) - 1) * (1ll << (n - z - o + c));
} else {
bool zero_in_ones = find( ones.begin(),  ones.end(), 0) !=  ones.end();
bool one_in_zeros = find(zeros.begin(), zeros.end(), 1) != zeros.end();
if (zero_in_ones and one_in_zeros) {
return (1ll << (z + o - c)) * (1ll << (n - z - o + c));
} else if (zero_in_ones) {
return (((1ll << (o - c)) - 1) * ((1ll << z) - 1) + 1 * (1ll << z)) * (1ll << (n - z - o + c));
} else if (one_in_zeros) {
return (((1ll << (z - c)) - 1) * ((1ll << o) - 1) + 1 * (1ll << o)) * (1ll << (n - z - o + c));
} else {
return (((1ll << c) - 1) * (1ll << (o - c + z - c)) + 1 * (1 * 1 + ((1ll << (z - c)) - 1) * ((1ll << (o - c)) - 1))) * (1ll << (n - z - o + c));
}
}
}