【Tarjan】UOJ#146 【NOIP2015】信息传递

题面在这里

Tarjan刷最小环……

示例程序：

#include<cstdio>#include<algorithm>using namespace std;inline char nc(){    static char buf[100000],*p1=buf,*p2=buf;    return p1==p2&&(p2=(p1=buf)+fread(buf,1,100000,stdin),p1==p2)?EOF:*p1++;}inline int red(){    int res=0,f=1;char ch=nc();    while (ch<'0'||'9'<ch) {if (ch=='-') f=-f;ch=nc();}    while ('0'<=ch&&ch<='9') res=res*10+ch-48,ch=nc();    return res*f;}const int maxn=200005;int n,ans;int tot,nxt[maxn],lnk[maxn],son[maxn];inline void add(int x,int y){    son[++tot]=y;nxt[tot]=lnk[x];lnk[x]=tot;}int dfn[maxn],low[maxn],stk[maxn],times;bool instk[maxn];void tarjan(int x){    dfn[x]=low[x]=++times;    stk[++stk[0]]=x;instk[x]=1;    for (int j=lnk[x];j;j=nxt[j])     if (!dfn[son[j]]) tarjan(son[j]),low[x]=min(low[x],low[son[j]]);else     if (instk[son[j]]) low[x]=min(low[x],dfn[son[j]]);    if (low[x]==dfn[x]){        int total=0;        while (stk[stk[0]+1]!=x)         instk[stk[stk[0]--]]=0,total++;        if (total>1) ans=min(ans,total);    }}int main(){    n=red();ans=1e9;    for (int i=1,x;i<=n;i++) x=red(),add(i,x);    for (int i=1;i<=n;i++)     if (!dfn[i]) tarjan(i);    printf("%d",ans);    return 0;}