[NOIP 2015] 信息传递

[题目链接]

           http://uoj.ac/problem/146

[算法]

         不难看出这是一棵基环外向森林，运行tarjan算法找出最小环即可
[代码]

        

#include<bits/stdc++.h>
using namespace std;
#define MAXN 200010
const int inf = 2e9;

struct edge
{
        int to,nxt;
} e[MAXN];
int i,n,ans,x,tot,timer,top,cnt;
int head[MAXN],size[MAXN],dfn[MAXN],low[MAXN],s[MAXN];
bool visited[MAXN],instack[MAXN];

namespace IO
{
    template <typename T> inline void read(T &x)
    {
        int f = 1; x = 0;
        char c = getchar();
        for (; !isdigit(c); c = getchar())
        {
            if (c == '-') f = -f;
        }
        for (; isdigit(c); c = getchar()) x = (x << 3) + (x << 1) + c - '0';
        x *= f;
    }
    template <typename T> inline void write(T x)
    {
        if (x < 0)
        {
            putchar('-');
            x = -x;
        }
        if (x > 9) write(x / 10);
        putchar(x % 10 + '0');
    }
    template <typename T> inline void writeln(T x)
    {
        write(x);
        puts("");
    }
} ;
inline void addedge(int u,int v)
{
        tot++;
        e[tot] = (edge){v,head[u]};
        head[u] = tot;
}
inline void tarjan(int u)
{
        int i,v;
        dfn[u] = low[u] = ++timer;
        instack[u] = true;
        s[++top] = u;
        for (i = head[u]; i; i = e[i].nxt)
        {
                v = e[i].to;
                if (!dfn[v])
                {
                        tarjan(v);
                        low[u] = min(low[u],low[v]);
                } else if (instack[u]) low[u] = min(low[u],dfn[v]);
        }
        if (low[u] == dfn[u])
        {
                cnt++;
                while (s[top + 1] != u)
                {
                        size[cnt]++;
                        top--;
                }
        }
}
int main() 
{
        
        IO :: read(n);
        for (i = 1; i <= n; i++)
        {
                IO :: read(x);
                addedge(i,x);
        }
        for (i = 1; i <= n; i++) 
        {
                if (!dfn[i])
                   tarjan(i);
        }
        ans = inf;
        for (i = 1; i <= cnt; i++) 
        {
                if (size[i] > 1)
                        ans = min(ans,size[i]);
        }
        IO :: writeln(ans);
        
        return 0;
    
}