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; }