diff --git a/best-time-to-buy-and-sell-stock/donghyeon95.java b/best-time-to-buy-and-sell-stock/donghyeon95.java
new file mode 100644
index 000000000..465df0408
--- /dev/null
+++ b/best-time-to-buy-and-sell-stock/donghyeon95.java
@@ -0,0 +1,24 @@
+class Solution {
+	public int maxProfit(int[] prices) {
+		int result = 0;
+		int maxProfit = 0;
+		int buyStock = prices[0];
+
+		// 한번 돌면서 나보다 작은 것이 나올 때까지 이익을 본다
+
+
+		for (int price: prices) {
+			// 나보다 작은 게 나오면 MAX 이익을 갱신하고 거기부서 다시 시작한다.
+			if (price < buyStock) {
+				result = Math.max(result, maxProfit);
+				maxProfit = 0;
+				buyStock = price;
+			} else {
+				maxProfit = Math.max(price - buyStock, maxProfit);
+			}
+		}
+
+		return Math.max(result, maxProfit);
+	}
+}
+
diff --git a/encode-and-decode-strings/donghyeon95.java b/encode-and-decode-strings/donghyeon95.java
new file mode 100644
index 000000000..6e512de3e
--- /dev/null
+++ b/encode-and-decode-strings/donghyeon95.java
@@ -0,0 +1,57 @@
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.Collectors;
+
+public class Solution {
+	/*
+	 * @param strs: a list of strings
+	 * @return: encodes a list of strings to a single string.
+	 */
+	String cDel = "&";
+	String sDel = ";";
+
+	// Encodes a list of strings to a single string
+	public String encode(List<String> strs) {
+		if (strs.isEmpty()) return null;
+
+		StringBuilder result = new StringBuilder();
+		for (int i =0; i<strs.size(); i++) {
+			String str = strs.get(i);
+			StringBuilder temp = new StringBuilder();
+			for (char c : str.toCharArray()) {
+				temp.append((int) c).append(cDel);
+			}
+
+			result.append(temp);
+			if (i != strs.size()-1)  result.append(sDel);
+		}
+		return result.toString();
+	}
+
+	// Decodes a single string to a list of strings
+	public List<String> decode(String str) {
+		if (str==null)
+			return new ArrayList<>();
+
+		List<String> result = new ArrayList<>();
+		String[] strs = str.split(sDel, -1);
+		for (String s : strs) {
+			if (s.isEmpty()) {
+				result.add("");
+				continue;
+			}
+			String[] chars = s.split(cDel);
+			String decoded = Arrays.stream(chars)
+				.filter(sr -> !sr.isEmpty())
+				.mapToInt(Integer::parseInt)
+				.mapToObj(ascii -> (char) ascii)
+				.map(String::valueOf)
+				.collect(Collectors.joining());
+			result.add(decoded);
+		}
+		return result;
+	}
+}
+
+
diff --git a/group-anagrams/donghyeon95.java b/group-anagrams/donghyeon95.java
new file mode 100644
index 000000000..7765cfd21
--- /dev/null
+++ b/group-anagrams/donghyeon95.java
@@ -0,0 +1,28 @@
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.List;
+
+class Solution {
+	public List<List<String>> groupAnagrams(String[] strs) {
+		// 문자열 정렬을 해서 같은 애들 모음을 주면 되지 않을까??
+		// 이럴 경우 정렬에 많은 시간을 소모
+
+		HashMap<String, List<String>> hm = new HashMap<>();
+		for (String str: strs) {
+			String arrangedStr = rearangeStr(str);
+			hm.putIfAbsent(arrangedStr, new ArrayList<>());
+			hm.get(arrangedStr).add(str);
+		}
+
+		return hm.values().stream().toList();
+	}
+
+	public String rearangeStr (String str) {
+		char[] chars = str.toCharArray();
+		Arrays.sort(chars);
+
+		return new String(chars);
+	}
+}
+
diff --git a/implement-trie-prefix-tree/donghyeon95.java b/implement-trie-prefix-tree/donghyeon95.java
new file mode 100644
index 000000000..170ced911
--- /dev/null
+++ b/implement-trie-prefix-tree/donghyeon95.java
@@ -0,0 +1,29 @@
+import java.util.HashMap;
+
+class Trie {
+	HashMap<String, Boolean> trie;
+	public Trie() {
+		trie = new HashMap<>();
+	}
+
+	public void insert(String word) {
+		StringBuilder sb = new StringBuilder();
+		for (char c: word.toCharArray()) {
+			sb.append(c);
+			trie.putIfAbsent(sb.toString(), false);
+		}
+		trie.put(sb.toString(), true);
+	}
+
+	public boolean search(String word) {
+		return trie.getOrDefault(word, false);
+	}
+
+	public boolean startsWith(String prefix) {
+		return trie.containsKey(prefix);
+	}
+}
+
+
+
+
diff --git a/word-break/donghyeon95.java b/word-break/donghyeon95.java
new file mode 100644
index 000000000..667ffc2fb
--- /dev/null
+++ b/word-break/donghyeon95.java
@@ -0,0 +1,41 @@
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.Collectors;
+
+class Solution {
+	public boolean wordBreak(String s, List<String> wordDict) {
+		return dfs(s, wordDict, new HashSet<>());
+	}
+
+	private boolean dfs(String s, List<String> wordDict, Set<String> dp) {
+		// 종료 조건: 문자열이 비어 있으면 성공
+		if (s.isEmpty()) return true;
+
+		// 중복 탐색 방지
+		if (dp.contains(s)) return false;
+
+		for (String word : wordDict) {
+			if (s.startsWith(word)) {
+				// 단어를 제거하고 재귀 호출
+				if (dfs(s.substring(word.length()), wordDict, dp)) {
+					return true;
+				}
+			}
+		}
+
+		// 단어를 제거하지 않고 넘어가는 경우도 탐색
+		dp.add(s); // 탐색이 실패한 상태 저장
+		return false;
+	}
+}
+
+
+