diff --git a/best-time-to-buy-and-sell-stock/daehyun99.py b/best-time-to-buy-and-sell-stock/daehyun99.py
new file mode 100644
index 0000000000..74d2624862
--- /dev/null
+++ b/best-time-to-buy-and-sell-stock/daehyun99.py
@@ -0,0 +1,13 @@
+class Solution:
+    def maxProfit(self, prices: List[int]) -> int:
+        l = 0
+        r = 0
+        profit = 0
+
+        while r < len(prices):
+            if prices[l] < prices[r]:
+                profit = max(prices[r]-prices[l], profit)
+            elif prices[l] > prices[r]:
+                l = r
+            r += 1
+        return profit
diff --git a/encode-and-decode-strings/daehyun99.py b/encode-and-decode-strings/daehyun99.py
new file mode 100644
index 0000000000..cdebf17b3c
--- /dev/null
+++ b/encode-and-decode-strings/daehyun99.py
@@ -0,0 +1,31 @@
+class Solution:
+
+    def encode(self, strs: List[str]) -> str:
+        encoded_strs = ""
+        for s in strs:
+            encoded_strs += str(len(s))
+            encoded_strs += "#"
+            for char in s:
+                encoded_strs += char
+        return encoded_strs
+                
+    def decode(self, s: str) -> List[str]:
+        print(s)
+        decoded_strs = []
+
+        idx = 0
+        while idx < len(s):
+            end = idx + 1
+            while s[end] != "#":
+                end += 1
+            length = s[idx:end]
+            idx = end
+            decoded_str = ""
+            for i in range(int(length)):
+                idx += 1
+                decoded_str += s[idx]
+            idx += 1
+            decoded_strs.append(decoded_str)
+        return decoded_strs
+
+
diff --git a/group-anagrams/daehyun99.py b/group-anagrams/daehyun99.py
new file mode 100644
index 0000000000..1fcdb475cf
--- /dev/null
+++ b/group-anagrams/daehyun99.py
@@ -0,0 +1,38 @@
+from collections import defaultdict
+class Solution:
+    def groupAnagrams(self, strs: List[str]) -> List[List[str]]:
+        groups = defaultdict(list)
+
+        for s in strs:
+            count = [0] * 26
+
+            for c in s:
+                count[ord(c)-ord('a')] += 1
+
+            groups[tuple(count)].append(s)
+        return list(groups.values())
+
+"""
+class Solution:
+    def groupAnagrams(self, strs: List[str]) -> List[List[str]]:
+        def decode(s, key, base):
+            for c in s:
+                key[ord(c) - base] += 1
+            result = ""
+            for k in key:
+                result += ' ' + str(k)
+            return result
+        keys = {}
+        base_key = [0 for i in range(26)]
+        base = ord("a")
+
+        for s in strs:
+            key = decode(s, base_key.copy(), base)
+            temp = keys.get(key, [])
+            temp.append(s)
+            keys[key] = temp
+        results = []
+        for val in keys.values():
+            results.append(val)
+        return results
+"""
diff --git a/implement-trie-prefix-tree/daehyun99.py b/implement-trie-prefix-tree/daehyun99.py
new file mode 100644
index 0000000000..c88af230f4
--- /dev/null
+++ b/implement-trie-prefix-tree/daehyun99.py
@@ -0,0 +1,43 @@
+class Trie:
+    def __init__(self):
+        self.root = dict()
+
+    def insert(self, word: str) -> None:
+        pointer = self.root
+        for char in word:
+            if char in pointer:
+                pointer = pointer[char]
+                continue
+            else:
+                pointer[char] = dict()
+                pointer = pointer[char]
+        pointer[0] = dict()
+
+    def search(self, word: str) -> bool:
+        pointer = self.root
+        for char in word:
+            if char in pointer:
+                pointer = pointer[char]
+                continue
+            else:
+                return False
+        if 0 in pointer:
+            return True
+        return False
+        
+    def startsWith(self, prefix: str) -> bool:
+        pointer = self.root
+        for char in prefix:
+            if char in pointer:
+                pointer = pointer[char]
+                continue
+            else:
+                return False
+        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/daehyun99.py b/word-break/daehyun99.py
new file mode 100644
index 0000000000..64ec706641
--- /dev/null
+++ b/word-break/daehyun99.py
@@ -0,0 +1,14 @@
+class Solution:
+    def wordBreak(self, s: str, wordDict: List[str]) -> bool:
+        pos = set()
+        pos.add(0)
+        len_s = len(s)
+
+        while len(pos) > 0 :
+            l = pos.pop()
+            for word in wordDict:
+                if s[l:].startswith(word):
+                    pos.add(l+len(word))
+            if l == len_s:
+                return True
+        return False