diff --git a/best-time-to-buy-and-sell-stock/yuseok89.py b/best-time-to-buy-and-sell-stock/yuseok89.py
new file mode 100644
index 0000000000..41e58cb30e
--- /dev/null
+++ b/best-time-to-buy-and-sell-stock/yuseok89.py
@@ -0,0 +1,18 @@
+# TC: O(N)
+# SC: O(1)
+class Solution:
+    def maxProfit(self, prices: List[int]) -> int:
+        min_until_now = prices[0]
+        max_profit = 0
+
+        for price in prices:
+            profit = price - min_until_now
+
+            if profit > max_profit:
+                max_profit = profit
+
+            if price < min_until_now:
+                min_until_now = price
+
+        return max_profit
+
diff --git a/group-anagrams/yuseok89.py b/group-anagrams/yuseok89.py
new file mode 100644
index 0000000000..bffab56efe
--- /dev/null
+++ b/group-anagrams/yuseok89.py
@@ -0,0 +1,12 @@
+# TC: O(N * LlogL)
+# SC: O(N * L)
+class Solution:
+    def groupAnagrams(self, strs: List[str]) -> List[List[str]]:
+
+        ans = defaultdict(list)
+
+        for s in strs:
+            ans[''.join(sorted(s))].append(s)
+
+        return list(ans.values())
+
diff --git a/implement-trie-prefix-tree/yuseok89.py b/implement-trie-prefix-tree/yuseok89.py
new file mode 100644
index 0000000000..de928a9957
--- /dev/null
+++ b/implement-trie-prefix-tree/yuseok89.py
@@ -0,0 +1,52 @@
+# TC: O(L)
+# SC: O(L * N)
+class TrieNode:
+    def __init__(self):
+        self.next = {}
+        self.is_end = False
+
+class Trie:
+
+    def __init__(self):
+        self.root = TrieNode()
+
+    def insert(self, word: str) -> None:
+        cur = self.root
+
+        for c in word:
+            if c not in cur.next:
+                cur.next[c] = TrieNode()
+
+            cur = cur.next[c]
+
+        cur.is_end = True
+
+    def search(self, word: str) -> bool:
+        cur = self.root
+
+        for c in word:
+            if c not in cur.next:
+                return False
+
+            cur = cur.next[c]
+
+        return cur.is_end
+
+    def startsWith(self, prefix: str) -> bool:
+        cur = self.root
+
+        for c in prefix:
+            if c not in cur.next:
+                return False
+
+            cur = cur.next[c]
+
+        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/yuseok89.py b/word-break/yuseok89.py
new file mode 100644
index 0000000000..e774129444
--- /dev/null
+++ b/word-break/yuseok89.py
@@ -0,0 +1,34 @@
+class Solution:
+    def wordBreak(self, s: str, wordDict: List[str]) -> bool:
+
+        word_set = set()
+        word_lens = set()
+
+        for word in wordDict:
+            word_set.add(word)
+            word_lens.add(len(word))
+
+        word_lens = sorted(word_lens)
+        visited = set()
+
+        def rec(cur):
+            if cur in visited:
+                return False
+
+            if len(cur) == 0:
+                return True
+
+            for l in word_lens:
+                if len(cur) < l:
+                    return False
+
+                if cur[0:l] in word_set:
+                    if rec(cur[l:]):
+                        return True
+
+            visited.add(cur)
+
+            return False
+
+        return rec(s)
+