update

CMakeLists.txt

@@ -19,5 +19,6 @@ add_executable(P1031 ${CMAKE_CURRENT_LIST_DIR}/P1031/main.cpp)
 add_executable(P1031_pro ${CMAKE_CURRENT_LIST_DIR}/P1031/pro.cpp)
 
 add_executable(P2661 ${CMAKE_CURRENT_LIST_DIR}/P2661/main.cpp)
+add_executable(P2661_video ${CMAKE_CURRENT_LIST_DIR}/P2661/video.cpp)
 
 add_executable(P2615 ${CMAKE_CURRENT_LIST_DIR}/P2615/P2615.cpp)

P2661/video.cpp

@@ -0,0 +1,61 @@
+#include <iostream>
+#include <istream>
+#include <ranges>
+#include <queue>
+#include <limits>
+#include <algorithm>
+
+using std::cin,std::cout;
+constexpr const auto range = std::ranges::views::iota;
+
+const int MAX_N = 2e5+5;
+int n;
+int next[MAX_N];
+int in_degree[MAX_N];
+std::queue<int> q;
+bool vis[MAX_N];
+int ans {std::numeric_limits<int>::max()};
+
+int main(){
+    std::iostream::sync_with_stdio(false),cin.tie(nullptr),cout.tie(0);
+
+    cin>>n;
+    for(const int i:range(1,n+1)){
+        cin>>next[i];
+        in_degree[next[i]]++;
+    }
+
+    const auto push_vis = [](const int n){
+        q.push(n);
+        vis[n]=true;
+    };
+    for(const int i:range(1,n+1)){
+        if(in_degree[i]==0){
+            push_vis(i);
+        }
+    }
+
+    while(q.empty()==false){
+        int front =  q.front();
+        q.pop();
+        in_degree[next[front]]--;
+        if(in_degree[next[front]]==0){
+            push_vis(next[front]);
+        }
+    }
+
+    for(int i:range(1,n+1)){
+        if(vis[i])continue;
+        const int start {i};
+        int now {start};
+        int ring_size {0};
+        do{
+            vis[now]=true;
+            ring_size++;
+            now = next[now];
+        }while(now!=start);
+        ans = std::min(ans,ring_size);
+    }
+
+    cout<<ans<<"\n";
+}