This problem does not have the optimal solution, but as long as each point is checked and the positions that point can reach are marked, so that they do not need to be searched again in the future.
If you want to find a cycle, you need to use another array to store it.
//====================================================================||
// ||
// ||
// Author : GCA ||
// 6AE7EE02212D47DAD26C32C0FE829006 ||
//====================================================================||
#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
#include <climits>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <cctype>
#include <utility>
using namespace std;
#ifdef ONLINE\_JUDGE
#define ll "%lld"
#else
#define ll "%I64d"
#endif
typedef unsigned int uint;
typedef long long int Int;
#define Set(a,s) memset(a,s,sizeof(a))
#define Write(w) freopen(w,"w",stdout)
#define Read(r) freopen(r,"r",stdin)
#define Pln() printf("\\n")
#define I\_de(x,n)for(int i=0;i<n;i++)printf("%d ",x\[i\]);Pln()
#define De(x)printf(#x"%d\\n",x)
#define For(i,x)for(int i=0;i<x;i++)
#define CON(x,y) x##y
#define Pmz(dp,nx,ny)for(int hty=0;hty<ny;hty++){for(int htx=0;htx<nx;htx++){\\
printf("%d ",dp\[htx\]\[hty\]);}Pln();}
#define M 50005
#define PII pair<int,int\>
#define PB push\_back
#define oo INT\_MAX
#define Set\_oo 0x3f
#define Is\_debug true
#define debug(...) if(Is\_debug)printf("DEBUG: "),printf(\_\_VA\_ARGS\_\_)
#define FOR(it,c) for(\_\_typeof((c).begin()) it=(c).begin();it!=(c).end();it++)
#define eps 1e-6
bool xdy(double x,double y){return x>y+eps;}
bool xddy(double x,double y){return x>y-eps;}
bool xcy(double x,double y){return x<y-eps;}
bool xcdy(double x,double y){return x<y+eps;}
int min3(int x,int y,int z){
int tmp=min(x,y);
return min(tmp,z);
}
int max3(int x,int y,int z){
int tmp=max(x,y);
return max(tmp,z);
}
int n;
int mz\[M\];
bool vis\[M\],vis2\[M\];
int t;
void dfs(int x){
vis\[x\]=vis2\[x\]=1;
if(mz\[x\]!=-1&&!vis2\[mz\[x\]\])dfs(mz\[x\]);
vis2\[x\]=0;
t++;
}
int main() {
ios\_base::sync\_with\_stdio(0);
int test,ff=0;
scanf("%d",&test);
while(test--){
Set(vis,0);
Set(mz,-1);
scanf("%d",&n);
for(int i=0;i<n;i++){
int x,y;
scanf("%d%d",&x,&y);
x--;y--;
mz\[x\]=y;
}
int ans=0,index=-1;
for(int i=0;i<n;i++){
t=0;
if(!vis\[i\])dfs(i);
if(t>ans){
ans=t;
index=i;
}
}
printf("Case %d: %d\\n",++ff,index+1);
}
}