diff --git a/CMakeLists.txt b/CMakeLists.txt
index a386677..85ca2c3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -5,12 +5,12 @@ add_compile_options(-Wall)
 
 set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
 
-if("${CMAKE_BUILD_TYPE}" STREQUAL "Release")
-    message("Is in [Release] mode")
-else()
-    message("Is in [NotRelease] mode")
+if(NOT CMAKE_BUILD_TYPE)
+    set(CMAKE_BUILD_TYPE Release)
 endif()
+message("Build type is [${CMAKE_BUILD_TYPE}]")
 
 
 project(alogrithm_2024)
diff --git a/P2661/main.cpp b/P2661/main.cpp
index 294989d..b1c16e5 100644
--- a/P2661/main.cpp
+++ b/P2661/main.cpp
@@ -1,3 +1,53 @@
+#include <algorithm>
+#include <iostream>
+#include <istream>
+#include <queue>
+
+using std::cin,std::cout;
+
+const int MAX_N {(int)2e5+5};
+int n;
+int next[MAX_N];
+int inDegree[MAX_N];
+bool vis[MAX_N];
+std::queue<int> q;
+int ans;
+
 int main(){
-    
+    std::iostream::sync_with_stdio(false),cin.tie(0),cout.tie(0);
+
+    std::cin>>n;
+    for(int i=1;i<=n;i++){
+        cin>>next[i];
+        inDegree[next[i]]++;
+    }
+    const auto push_vis = [](int n){
+        q.push(n);
+        vis[n]=true;
+    };
+    for(int i=1;i<=n;i++){
+        if(inDegree[i]<=0){
+            push_vis(i);
+        }
+    }
+    while(q.empty()==false){
+        int front = q.front();
+        q.pop();
+        if(--inDegree[next[front]]<=0){
+            push_vis(next[front]);
+        }
+    }
+    for(int i=1;i<=n;i++){
+        if(vis[i])continue;
+        int loop_size {0};
+        int start {i};
+        int now {start};
+        do{
+            vis[now]=true;
+            now=next[now];
+            loop_size++;
+        }while(now!=start);
+        ans=std::max(ans,loop_size);
+    }
+    std::cout<<ans<<"\n";
 }
\ No newline at end of file