diff --git a/best-time-to-buy-and-sell-stock/sangbeenmoon.py b/best-time-to-buy-and-sell-stock/sangbeenmoon.py
new file mode 100644
index 0000000000..2fe1894930
--- /dev/null
+++ b/best-time-to-buy-and-sell-stock/sangbeenmoon.py
@@ -0,0 +1,13 @@
+# TC : O(n)
+# SC : O(1)
+
+class Solution:
+    def maxProfit(self, prices: List[int]) -> int:
+        answer = 0
+        mm = 10001
+
+        for price in prices:
+            mm = min(mm, price)
+            answer = max(answer, price - mm)
+        
+        return answer
diff --git a/encode-and-decode-strings/sangbeenmoon.py b/encode-and-decode-strings/sangbeenmoon.py
new file mode 100644
index 0000000000..7478d851e0
--- /dev/null
+++ b/encode-and-decode-strings/sangbeenmoon.py
@@ -0,0 +1,43 @@
+class Solution:
+    """
+    @param: strs: a list of strings
+    @return: encodes a list of strings to a single string.
+    """
+    def encode(self, strs):
+        # write your code here
+        result = ""
+
+        delimiter = ":;"
+
+        for i, str in enumerate(strs):
+            result = result + str
+            if i != len(strs):
+                result = result + delimiter
+        
+        print(result)
+
+        return result
+            
+    """
+    @param: str: A string
+    @return: decodes a single string to a list of strings
+    """
+    def decode(self, str):
+        result = []
+
+        i = 0 
+        delimiter = ":;"
+
+        while i < len(str):
+
+            j = i
+            while j < len(str) - 1:
+                if str[j:j+2] == delimiter:
+                    result.append(str[i:j])
+                    break
+                else:
+                    j = j + 1
+
+            i = j + 2
+        
+        return result
diff --git a/group-anagrams/sangbeenmoon.py b/group-anagrams/sangbeenmoon.py
new file mode 100644
index 0000000000..72a260e00a
--- /dev/null
+++ b/group-anagrams/sangbeenmoon.py
@@ -0,0 +1,35 @@
+# TC : O(m^2 * n) m : len(strs), n : len(strs[0]) 
+# SC : O(m * n)
+
+class Solution:
+    def groupAnagrams(self, strs: List[str]) -> List[List[str]]:
+        sorted_strs = []
+        for str in strs:
+            sorted_strs.append(sorted(str))
+        
+        answer = []
+        
+        i = 0
+
+        visited = [False] * 10001
+
+        while i < len(strs):
+            if visited[i]:
+                i = i + 1
+                continue
+
+            sub_answer = []
+            target = sorted_strs[i]
+            sub_answer.append(strs[i])
+
+            for j in range(i+1, len(strs)):
+                if sorted_strs[j] == target:
+                    visited[j] = True
+                    sub_answer.append(strs[j])
+            
+            i = i + 1
+            answer.append(sub_answer)
+        
+        return answer
+            
+            
diff --git a/implement-trie-prefix-tree/sangbeenmoon.java b/implement-trie-prefix-tree/sangbeenmoon.java
new file mode 100644
index 0000000000..67c598f6b3
--- /dev/null
+++ b/implement-trie-prefix-tree/sangbeenmoon.java
@@ -0,0 +1,57 @@
+import java.util.HashMap;
+import java.util.Map;
+
+class Trie {
+
+    Map<Character, Trie> map;
+    boolean isEnd = false;
+
+    public Trie() {
+        map = new HashMap<>();
+    }
+
+    public void insert(String word) {
+        Trie trie;
+        if (map.containsKey(word.charAt(0))) {
+            trie = map.get(word.charAt(0));
+        } else {
+            trie = new Trie();
+        }
+        map.put(word.charAt(0), trie);
+
+        if (word.length() == 1) {
+            map.get(word.charAt(0)).isEnd = true;
+            return;
+        }
+
+        trie.insert(word.substring(1));
+    }
+
+    public boolean search(String word) {
+        if (map.containsKey(word.charAt(0))) {
+            if (word.length() == 1) {
+                return map.get(word.charAt(0)).isEnd;
+            }
+            return map.get(word.charAt(0)).search(word.substring(1));
+        }
+        return false;
+    }
+
+    public boolean startsWith(String prefix) {
+        if (map.containsKey(prefix.charAt(0))) {
+            if (prefix.length() == 1) {
+                return true;
+            }
+            return map.get(prefix.charAt(0)).startsWith(prefix.substring(1));
+        }
+        return false;
+    }
+}
+
+/**
+ * Your Trie object will be instantiated and called as such:
+ * Trie obj = new Trie();
+ * obj.insert(word);
+ * boolean param_2 = obj.search(word);
+ * boolean param_3 = obj.startsWith(prefix);
+ */
diff --git a/word-break/sangbeenmoon.py b/word-break/sangbeenmoon.py
new file mode 100644
index 0000000000..d580913361
--- /dev/null
+++ b/word-break/sangbeenmoon.py
@@ -0,0 +1,29 @@
+# 실패한 풀이.
+# AC 를 받기는 했으나 test case 가 좀 더 촘촘했다면 TLE 에 걸렸을 것임.
+# string 이 아닌 index 로 memoization 을 하는 걸 떠올려보자.
+
+class Solution:
+    answer = False
+    visited = {}
+    def wordBreak(self, s: str, wordDict: List[str]) -> bool:
+        
+        self.answer = False
+        self.visited = {}
+        self.go(0,s,wordDict)
+
+        return self.answer
+
+    def go(self, i:int, s: str, wordDict: List[str]):
+        if i >= (len(s)):
+            self.answer = True
+            return
+        
+        for word in wordDict:
+            if i + len(word) > len(s):
+                continue
+
+            if word == s[i:i + len(word)]:
+                if not s[i + len(word) : ] in self.visited:
+                    self.go(i + len(word), s, wordDict)
+        
+        self.visited[s[i:]] = False