diff --git a/Basic_Of_Algorithm/src/interestingAlgorithm/Collatz_Graph.java b/Basic_Of_Algorithm/src/interestingAlgorithm/Collatz_Graph.java
new file mode 100644
index 0000000..2291b9a
--- /dev/null
+++ b/Basic_Of_Algorithm/src/interestingAlgorithm/Collatz_Graph.java
@@ -0,0 +1,31 @@
+package interestingAlgorithm;
+
+import java.util.Scanner;
+
+public class Collatz_Graph {
+
+	public static void main(String[] args) {
+		
+		Scanner scan = new Scanner(System.in);
+		
+		int n = scan.nextInt();
+		
+		
+		while(n!=1) {
+			
+			if(n%2==0) n /=2;
+			else n = (n*3) + 1;
+			
+			for(int i =0;i<n;i++) {
+				System.out.print("0");
+			}
+			
+			System.out.println();
+			
+			
+		}
+	
+		
+		
+	}
+}
diff --git a/Basic_Of_Algorithm/src/interestingAlgorithm/HashMapTest.java b/Basic_Of_Algorithm/src/interestingAlgorithm/HashMapTest.java
new file mode 100644
index 0000000..9d3a36c
--- /dev/null
+++ b/Basic_Of_Algorithm/src/interestingAlgorithm/HashMapTest.java
@@ -0,0 +1,62 @@
+package interestingAlgorithm;
+
+import java.util.HashMap;
+import java.util.ArrayList;
+
+public class HashMapTest {
+
+	public static void main(String[] args) {
+		
+		
+		String [] record = {"Enter uid1234 Muzi", "Enter uid4567 Prodo","Leave uid1234","Enter uid1234 Prodo","Change uid4567 Ryan"};
+		String []answer;
+		
+		
+		HashMap<String,String>user = new HashMap<>();
+		
+		
+		for(String str : record) {
+			
+			String[]temp = str.split(" ");
+			
+			String action =temp[0];
+			String id = temp[1];
+			
+			if(action.equals("Leave"))continue;
+			
+			user.put(id, temp[2]);
+			
+			
+		}
+		
+		ArrayList<String>answerList = new ArrayList<>();
+		
+		for(String str : record) {
+			
+			String[]temp = str.split(" ");
+			
+			String action = temp[0];
+			String id = temp[1];
+			
+			if(action.equals("Change"))continue;
+			
+			String nick = user.get(id);
+			
+			String ans = nick +"님이 " + (action.equals("Enter") ? "들어왔습니다." :  "나갔습니다.");
+			
+			answerList.add(ans);
+			
+			
+		}
+		
+		
+		answer = new String[answerList.size()];
+		
+		for(int i=0;i<answerList.size();i++) answer[i] = answerList.get(i);
+		
+		for(String str : answer) System.out.println(str);
+		
+		
+		
+	}
+}
diff --git a/Basic_Of_Algorithm/src/interestingAlgorithm/collatz_conjecture.java b/Basic_Of_Algorithm/src/interestingAlgorithm/collatz_conjecture.java
new file mode 100644
index 0000000..af7217b
--- /dev/null
+++ b/Basic_Of_Algorithm/src/interestingAlgorithm/collatz_conjecture.java
@@ -0,0 +1,21 @@
+package interestingAlgorithm;
+
+public class collatz_conjecture {
+
+	public static void main(String[] args) {
+		
+		int n = 6;
+		int answer = 0;
+		
+		while(true) {
+			
+			if(n%2==0) n /= 2;
+			else n = (n*3) + 1;
+			
+			answer++;
+			
+			System.out.println(n);
+		}
+		
+	}
+}