diff --git a/best-time-to-buy-and-sell-stock/devyejin.py b/best-time-to-buy-and-sell-stock/devyejin.py
new file mode 100644
index 0000000000..21f3781b47
--- /dev/null
+++ b/best-time-to-buy-and-sell-stock/devyejin.py
@@ -0,0 +1,16 @@
+# time complexity : O(n)
+# space complexity : O(1)
+from typing import List
+
+
+class Solution:
+    def maxProfit(self, prices: List[int]) -> int:
+        n = len(prices)
+        min_price = prices[0]
+        max_profit = 0
+
+        for i in range(1, n):
+            min_price = min(min_price, prices[i])
+            max_profit = max(max_profit, prices[i] - min_price)
+
+        return max_profit
diff --git a/encode-and-decode-strings/devyejin.py b/encode-and-decode-strings/devyejin.py
new file mode 100644
index 0000000000..dcb9bfe09c
--- /dev/null
+++ b/encode-and-decode-strings/devyejin.py
@@ -0,0 +1,33 @@
+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 = ""
+        for s in strs:
+            result += str(len(s)) + "@" + s
+        return result
+
+    """
+    @param: str: A string
+    @return: decodes a single string to a list of strings
+    """
+
+    def decode(self, str):
+        # write your code here
+        result = []
+        i = 0
+        while i < len(str):
+            j = i
+            # 시작점 아닌 경우
+            while str[j] != "@":
+                j += 1
+            # 시작점인 경우
+            length = int(str[i:j])
+            word = str[j + 1: j + 1 + length]
+            result.append(word)
+            i = j + 1 + length
+        return result
diff --git a/implement-trie-prefix-tree/devyejin.py b/implement-trie-prefix-tree/devyejin.py
new file mode 100644
index 0000000000..6a335881d4
--- /dev/null
+++ b/implement-trie-prefix-tree/devyejin.py
@@ -0,0 +1,61 @@
+class TrieNode:
+    def __init__(self):
+        self.isEnd = False
+        self.links = {}
+
+
+class Trie:
+
+    def __init__(self):
+        self._root = TrieNode()
+
+    def _recurAdd(self, node: TrieNode, word: str) -> None:
+        if not word:
+            node.isEnd = True
+            return
+
+        ch = word[0]
+        # 부모 노드의 자식에 있는지 확인
+        next_link = node.links.get(ch)
+
+        if not next_link:
+            node.links[ch] = TrieNode()
+            next_link = node.links[ch]
+
+        self._recurAdd(next_link, word[1:])
+
+    def insert(self, word: str) -> None:
+        if not word:
+            return
+
+        self._recurAdd(self._root, word)
+
+    def _recurSearch(self, node: TrieNode, word: str) -> bool:
+        if not word:
+            return node.isEnd
+
+        ch = word[0]
+        next_link = node.links.get(ch)
+        if next_link:
+            return self._recurSearch(next_link, word[1:])
+        return False
+
+    def search(self, word: str) -> bool:
+        if not word:
+            return False
+
+        return self._recurSearch(self._root, word)
+
+    def startsWith(self, prefix: str) -> bool:
+        node = self._root
+        for ch in prefix:
+            if ch not in node.links:
+                return False
+            node = node.links[ch]
+        return True
+
+# Your Trie object will be instantiated and called as such:
+# obj = Trie()
+# obj.insert(word)
+# param_2 = obj.search(word)
+# param_3 = obj.startsWith(prefix)
diff --git a/word-break/devyejin.py b/word-break/devyejin.py
new file mode 100644
index 0000000000..26a9ca84a1
--- /dev/null
+++ b/word-break/devyejin.py
@@ -0,0 +1,24 @@
+from typing import List
+
+
+class Solution:
+    def wordBreak(self, s: str, wordDict: List[str]) -> bool:
+        def dfs(subs: str):
+            if subs in memo:
+                return memo[subs]
+
+            if not subs:
+                return True
+
+            for word in wordDict:
+                if subs.startswith(word):
+                    if dfs(subs[len(word):]):
+                        memo[subs] = True
+                        return True
+
+            memo[subs] = False
+            return False
+
+        memo = {}
+        return dfs(s)
+