diff --git a/HyoJeong/4week_daily/Day0.java b/HyoJeong/4week_daily/Day0.java
new file mode 100644
index 0000000..b99cc01
--- /dev/null
+++ b/HyoJeong/4week_daily/Day0.java
@@ -0,0 +1,21 @@
+import java.util.*;
+
+public class counting {
+    public static boolean main(String[] args) {
+        Scanner scanner = new Scanner(System.in);
+        String s = scanner.nextLine();
+
+        int countp = 0;
+        int county = 0;
+
+        for(char c : s.toCharArray())
+        {
+            if (c == 'p'||c == 'P')
+                countp++;
+            else if(c == 'y'|| c == 'Y')
+                county++;
+
+        }
+        return countp == county;
+    }
+}
diff --git a/HyoJeong/4week_daily/Day09.java b/HyoJeong/4week_daily/Day09.java
new file mode 100644
index 0000000..c5952df
--- /dev/null
+++ b/HyoJeong/4week_daily/Day09.java
@@ -0,0 +1,27 @@
+import java.util.*;
+
+public class Main {
+    public static void main(String[] args){
+        //Set <String> PW = new HashSet<String>();
+        ArrayList <String> PW = new ArrayList<String>();
+        String reverse = "";
+
+        Scanner scanner = new Scanner(System.in);
+        int n = Integer.parseInt(scanner.nextLine());
+
+        for (int i = 0; i<n; i++){
+            String password = scanner.nextLine();
+            PW.add(password);
+            reverse = new StringBuilder(password).reverse().toString();
+
+            if(PW.contains(reverse)) {
+                int length = password.length();
+                char midChar = password.charAt(length / 2);
+
+                System.out.println(length + " " + midChar);
+                return;
+            }
+        }
+        scanner.close();
+    }
+}
diff --git a/HyoJeong/4week_daily/Day10.java b/HyoJeong/4week_daily/Day10.java
new file mode 100644
index 0000000..4e4daf7
--- /dev/null
+++ b/HyoJeong/4week_daily/Day10.java
@@ -0,0 +1,18 @@
+import java.util.*;
+
+class Solution {
+    public int solution(int[] nums) {
+        int answer = 0;
+        int count = 0;
+
+        HashMap <Integer, Integer> map = new HashMap <>();
+
+        //각 폰켓몬 종류를 HashMap에 저장
+        for(int i : nums){
+            map.put(i, 1);
+        }
+
+        //폰켓몬 종류와 N/2 중 더 작은 값을 출력
+        return Math.min(map.size(), nums.length / 2);
+    }
+}
diff --git a/HyoJeong/4week_daily/Day11.java b/HyoJeong/4week_daily/Day11.java
new file mode 100644
index 0000000..ccd4173
--- /dev/null
+++ b/HyoJeong/4week_daily/Day11.java
@@ -0,0 +1,25 @@
+import java.util.*;
+
+class Solution {
+    public String solution(String[] participant, String[] completion) {
+        String answer = "";
+        HashSet <String> set = new HashSet<>();
+
+        for (String s : participant){
+            if (!set.contains(s)){
+                set.add(s);
+            }
+            else
+                set.remove(s);
+        }
+        for (String s : completion){
+            if(!set.contains(s)){
+                set.add(s);
+            }
+            else
+                set.remove(s);
+        }
+        answer = String.join("", set);
+        return answer;
+    }
+}
diff --git a/HyoJeong/4week_daily/Day12.java b/HyoJeong/4week_daily/Day12.java
new file mode 100644
index 0000000..7535667
--- /dev/null
+++ b/HyoJeong/4week_daily/Day12.java
@@ -0,0 +1,44 @@
+import java.util.*;
+
+public class Main {
+    public static void main(String[] args) {
+        Scanner scanner = new Scanner(System.in);
+        int n = Integer.parseInt(scanner.nextLine());
+
+        Stack<Integer> S = new Stack<>();
+
+        for (int k = 0; k < n; k++) {
+            String function = scanner.next();
+            switch (function) {
+                case "push":
+                    int i = Integer.parseInt(scanner.next());
+                    S.push(i);
+                    break;
+                case "pop":
+                    if (S.isEmpty())
+                        System.out.println(-1);
+                    else {
+                        System.out.println(S.pop());
+                    }
+                    break;
+                case "size":
+                    System.out.println(S.size());
+                    break;
+                case "empty":
+                    if (S.isEmpty())
+                        System.out.println(1);
+                    else
+                        System.out.println(0);
+                    break;
+                case "top":
+                    if (S.isEmpty())
+                        System.out.println(-1);
+                    else {
+                        System.out.println(S.peek());
+                    }
+                    break;
+            }
+        }
+        scanner.close();
+    }
+}
diff --git a/HyoJeong/4week_daily/Day13.java b/HyoJeong/4week_daily/Day13.java
new file mode 100644
index 0000000..b24e9fb
--- /dev/null
+++ b/HyoJeong/4week_daily/Day13.java
@@ -0,0 +1,24 @@
+import java.util.*;
+
+public class Main {
+    public static void main(String[] args) {
+        Scanner scanner = new Scanner(System.in);
+        int n = Integer.parseInt(scanner.nextLine());
+
+        Stack<String> S = new Stack<>();
+
+        for (int i = 0; i < n; i++) {
+            String sentence = scanner.nextLine();
+            String[] RS = sentence.split(" ");
+            for (String rs : RS){
+                S.push(rs);
+            }
+            System.out.print("Case #" + (i + 1) + ": ");
+            while (S.isEmpty() == false){
+                System.out.print(S.pop() + " ");
+            }
+            System.out.println();
+        }
+    }
+
+}
diff --git a/HyoJeong/4week_daily/Day14.java b/HyoJeong/4week_daily/Day14.java
new file mode 100644
index 0000000..d757dad
--- /dev/null
+++ b/HyoJeong/4week_daily/Day14.java
@@ -0,0 +1,49 @@
+import java.util.*;
+
+public class Main {
+    public static void main(String[] args) {
+        Scanner scanner = new Scanner(System.in);
+        int n = Integer.parseInt(scanner.nextLine());
+
+        LinkedList <Integer> Q = new LinkedList<>();
+
+        for (int k = 0; k < n; k++) {
+            String function = scanner.next();
+            switch (function) {
+                case "push":
+                    int i = Integer.parseInt(scanner.next());
+                    Q.addLast(i);
+                    break;
+                case "pop":
+                    if (Q.isEmpty())
+                        System.out.println(-1);
+                    else {
+                        System.out.println(Q.removeFirst());
+                    }
+                    break;
+                case "size":
+                    System.out.println(Q.size());
+                    break;
+                case "empty":
+                    if (Q.isEmpty())
+                        System.out.println(1);
+                    else
+                        System.out.println(0);
+                    break;
+                case "front":
+                    if (Q.isEmpty())
+                        System.out.println(-1);
+                    else
+                        System.out.println(Q.getFirst());
+                    break;
+                case "back":
+                    if (Q.isEmpty())
+                        System.out.println(-1);
+                    else
+                        System.out.println(Q.getLast());
+                    break;
+            }
+        }
+        scanner.close();
+    }
+}
diff --git a/HyoJeong/4week_daily/readme.txt b/HyoJeong/4week_daily/readme.txt
new file mode 100644
index 0000000..8b13789
--- /dev/null
+++ b/HyoJeong/4week_daily/readme.txt
@@ -0,0 +1 @@
+
diff --git a/HyoJeong/SchoolStudy/readme.txt b/HyoJeong/SchoolStudy/readme.txt
new file mode 100644
index 0000000..8b13789
--- /dev/null
+++ b/HyoJeong/SchoolStudy/readme.txt
@@ -0,0 +1 @@
+
diff --git a/Main7_1.java b/Main7_1.java
new file mode 100644
index 0000000..3bee07b
--- /dev/null
+++ b/Main7_1.java
@@ -0,0 +1,23 @@
+import java.util.*;
+
+public class Main {
+    public static void main(String[] args){
+        Scanner sc = new Scanner(System.in);
+        String[] words = new String[5];
+        String[] result = {};
+
+        for (int i=0; i<5; i++){
+            words[i] = sc.nextLine();
+        }
+
+        StringBuilder sb = new StringBuilder();
+        for (int x=0; x<15; x++){
+            for (int y=0; y<5; y++){
+                if (x<words[y].length())
+                    sb.append(words[y].charAt(x));
+            }
+        }
+
+        System.out.print(sb);
+    }
+}
diff --git a/Main7_2.java b/Main7_2.java
new file mode 100644
index 0000000..07a8cf9
--- /dev/null
+++ b/Main7_2.java
@@ -0,0 +1,31 @@
+import java.util.*;
+
+public class Main {
+    public static void main(String[] args) {
+        Scanner sc = new Scanner(System.in);
+        int scan = sc.nextInt();
+        sc.nextLine();
+
+        //StringBuilder 배열 생성, but 각 요소는 null (생성만 해 둔 상태로 StringBuilder 객체 가리키지 X)
+        StringBuilder[] result = new StringBuilder[scan];
+
+        for (int i = 0; i < scan; i++) {
+            String input = sc.nextLine();
+            String[] example = input.split(" ");
+
+            int k = Integer.parseInt(example[0]);
+            String word = example[1];
+            result[i] = new StringBuilder(); //각 배열 요소에 할당
+
+            for (int a = 0; a< word.length(); a++){
+                for (int b = 0; b < k; b++) {
+                    result[i].append(word.charAt(a));
+                }
+            }
+        }
+
+        for(StringBuilder s : result){
+            System.out.println(s);
+        }
+    }
+}
diff --git a/Solution7_3.java b/Solution7_3.java
new file mode 100644
index 0000000..6d0a6b3
--- /dev/null
+++ b/Solution7_3.java
@@ -0,0 +1,24 @@
+import java.util.*;
+
+class Solution {
+    public int[] solution(int[] array, int[][] commands) {
+        int[] result = new int[commands.length];
+
+        for (int i = 0; i < commands.length; i++) {
+            int start = commands[i][0] - 1; //자르기 시작할 위치
+            int end = commands[i][1]; //자르기 끝낼 위치
+            int k = commands[i][2]; //k번째
+
+            //부분 배열 생성
+            int[] slicedArray = Arrays.copyOfRange(array, start, end);
+
+            //정렬
+            Arrays.sort(slicedArray);
+
+            //k번째 원소 출력
+            result[i] = slicedArray[k - 1];
+        }
+
+        return result;
+    }
+}
\ No newline at end of file