[BZOJ1589] [Usaco2008 Dec]Trick or Treat on the Farm 采集糖果（tarjan缩点 + 记忆化搜索）

传送门

先用tarjan缩点，再记忆话搜索一下

#include <stack>
#include <cstdio>
#include <cstring>
#include <iostream>
#define N 100001
#define min(x, y) ((x) < (y) ? (x) : (y))

int n, cnt, rp, tot, cnt1;
int head1[N], to1[N], next1[N], head[N], to[N], next[N], dfn[N], low[N], belong[N], ans[N], a[N];
bool ins[N];
std::stack <int> s;

inline int read()
{
	int x = 0, f = 1;
	char ch = getchar();
	for(; !isdigit(ch); ch = getchar()) if(ch == '-') f = -1;
	for(; isdigit(ch); ch = getchar()) x = (x << 1) + (x << 3) + ch - '0';
	return x * f;
}

inline void add(int x, int y)
{
	to[cnt] = y;
	next[cnt] = head[x];
	head[x] = cnt++;
}

inline void add1(int x, int y)
{
	to1[cnt1] = y;
	next1[cnt1] = head1[x];
	head1[x] = cnt1++;
}

inline void tarjan(int u)
{
	int i, v;
	dfn[u] = low[u] = ++rp;
	ins[u] = 1;
	s.push(u);
	for(i = head[u]; i ^ -1; i = next[i])
	{
		v = to[i];
		if(!dfn[v])
		{
			tarjan(v);
			low[u] = min(low[u], low[v]);
		}
		else if(ins[v]) low[u] = min(low[u], dfn[v]);
	}
	if(low[u] == dfn[u])
	{
		tot++;
		do
		{
			v = s.top();
			s.pop();
			ins[v] = 0;
			belong[v] = tot;
		} while(u ^ v);
	}
}

inline int dfs(int u)
{
	int i, v;
	if(ans[u])
		return ans[u];
	for(i = head1[u]; i ^ -1; i = next1[i])
	{
		v = to1[i];
		ans[u] = dfs(v);
	}
	return ans[u] += a[u];
}

int main()
{
	int i, x, u, v;
	memset(head, -1, sizeof(head));
	memset(head1, -1, sizeof(head1));
	n = read();
	for(i = 1; i <= n; i++)
	{
		x = read();
		add(i, x);
	}
	for(i = 1; i <= n; i++)
		if(!dfn[i])
			tarjan(i);
	for(i = 1; i <= n; i++) a[belong[i]]++;
	for(u = 1; u <= n; u++)
		for(i = head[u]; i ^ -1; i = next[i])
		{
			v = to[i];
			if(belong[u] ^ belong[v])
				add1(belong[u], belong[v]);
		}
	for(i = 1; i <= n; i++)
		printf("%d
", dfs(belong[i]));
	return 0;
}