diff --git a/combination-sum/bhyun-kim.py b/combination-sum/bhyun-kim.py
new file mode 100644
index 000000000..5303cd5b5
--- /dev/null
+++ b/combination-sum/bhyun-kim.py
@@ -0,0 +1,49 @@
+"""
+39. Combination Sum
+https://leetcode.com/problems/combination-sum/
+
+Solution
+    To solve this problem, we can use backtracking.
+    The idea is to explore all possible combinations starting from each candidate.
+    
+    - We can sort the candidates to avoid duplicates.
+    - We can create a helper function that takes the remaining target, the current combination, and the start index.
+    - If the remaining target is 0, we have found a valid combination, so we add it to the result.
+    - If the remaining target is negative, we return.
+    - We iterate through the candidates starting from the start index.
+    - We add the current candidate to the combination and recursively call the helper function with the updated target and combination.
+    - After the recursive call, we remove the current candidate from the combination.
+
+Time complexity: O(2^n)
+    - In the worst case, we explore all possible combinations.
+    - For each candidate, we have two choices: include it or exclude it.
+
+Space complexity: O(n)
+    - The recursive call stack has a maximum depth of n.
+"""
+
+from typing import List
+
+
+class Solution:
+    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
+        def backtrack(remain, comb, start):
+            if remain == 0:
+                result.append(list(comb))
+                return
+            elif remain < 0:
+                return
+
+            for i in range(start, len(candidates)):
+                current_candidate = candidates[i]
+                if current_candidate > remain:
+                    break
+
+                comb.append(current_candidate)
+                backtrack(remain - current_candidate, comb, i)
+                comb.pop()
+
+        candidates.sort()
+        result = []
+        backtrack(target, [], 0)
+        return result
diff --git a/construct-binary-tree-from-preorder-and-inorder-traversal/bhyun-kim.py b/construct-binary-tree-from-preorder-and-inorder-traversal/bhyun-kim.py
new file mode 100644
index 000000000..83a993483
--- /dev/null
+++ b/construct-binary-tree-from-preorder-and-inorder-traversal/bhyun-kim.py
@@ -0,0 +1,63 @@
+"""
+105. Construct Binary Tree from Preorder and Inorder Traversal
+https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/
+"""
+
+from typing import List, Optional
+
+
+# Definition for a binary tree node.
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+
+"""
+Solution
+    To solve this problem, we can use a recursive approach. 
+    We can use a helper function that takes the left and right index of the inorder array.
+    The helper function will create a root node with the value of the current preorder index.
+    Then, it will recursively call itself with the left and right index of the left and right subtree.
+
+    - Create a dictionary that maps the value of the inorder array to its index.
+    - Create a variable to keep track of the current preorder index.
+    - Create a helper function that takes the left and right index of the inorder array.
+    - In the helper function, if the left index is greater than the right index, return None.
+    - Create a root node with the value of the current preorder index.
+    - Increment the preorder index.
+    - Recursively call the helper function with the left and right index of the left and right subtree.
+    - Return the root node.
+
+Time Complexity : O(n)
+    - The helper function is called n times.
+    - The dictionary lookup is O(1).
+
+Space Complexity : O(n)
+    - The dictionary has n elements.
+    - The recursive call stack has n elements.
+"""
+
+
+class Solution:
+    def buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:
+        inorder_index_map = {val: idx for idx, val in enumerate(inorder)}
+
+        self.preorder_index = 0
+
+        def array_to_tree(left, right):
+            if left > right:
+                return None
+
+            root_value = preorder[self.preorder_index]
+            self.preorder_index += 1
+
+            root = TreeNode(root_value)
+
+            root.left = array_to_tree(left, inorder_index_map[root_value] - 1)
+            root.right = array_to_tree(inorder_index_map[root_value] + 1, right)
+
+            return root
+
+        return array_to_tree(0, len(inorder) - 1)
diff --git a/implement-trie-prefix-tree/bhyun-kim.py b/implement-trie-prefix-tree/bhyun-kim.py
new file mode 100644
index 000000000..8624fc137
--- /dev/null
+++ b/implement-trie-prefix-tree/bhyun-kim.py
@@ -0,0 +1,46 @@
+"""
+208. Implement Trie (Prefix Tree)
+https://leetcode.com/problems/implement-trie-prefix-tree/
+
+Solution: 
+    - Initialize the Trie class with an empty list of words.
+    - Insert a word by appending it to the list of words.
+    - Search for a word by checking if it is in the list of words.
+    - Check if a prefix exists by iterating through the list of words and checking if any word starts with the prefix.
+
+Tiem complexity: 
+    - Insert: O(1)
+        - Appending to a list is O(1)
+    - Search: O(n)
+        - Searching for an element in a list is O(n)
+    - StartsWith: O(n)
+        - Iterating through a list is O(n)
+
+Space complexity: O(n)
+    - The list of words may have all the words in the trie.
+"""
+
+
+class Trie:
+    def __init__(self):
+        self.words = []
+
+    def insert(self, word: str) -> None:
+        self.words.append(word)
+
+    def search(self, word: str) -> bool:
+        return word in self.words
+
+    def startsWith(self, prefix: str) -> bool:
+        for word in self.words:
+            if word.startswith(prefix):
+                return True
+
+        return False
+
+
+# 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/kth-smallest-element-in-a-bst/bhyun-kim.py b/kth-smallest-element-in-a-bst/bhyun-kim.py
new file mode 100644
index 000000000..9cf26737d
--- /dev/null
+++ b/kth-smallest-element-in-a-bst/bhyun-kim.py
@@ -0,0 +1,40 @@
+"""
+230. Kth Smallest Element in a BST
+https://leetcode.com/problems/kth-smallest-element-in-a-bst/
+
+Solution:
+    To solve this problem, we can use an in-order traversal of the binary search tree.
+    We can create a helper function that performs an in-order traversal of the tree and returns a list of the elements in sorted order.
+    Then, we can return the k-th element from the list.
+
+Time complexity: O(n)
+    - The in-order traversal visits each node once.
+    - The list concatenation is O(n).
+
+Space complexity: O(n)
+    - The list of elements has n elements.
+"""
+
+
+from typing import Optional
+
+
+# Definition for a binary tree node.
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+
+class Solution:
+    def kthSmallest(self, root: Optional[TreeNode], k: int) -> int:
+        return self.inOrderSearch(root)[k - 1]
+
+    def inOrderSearch(self, root):
+        output = []
+        if root:
+            output += self.inOrderSearch(root.left)
+            output += [root.val]
+            output += self.inOrderSearch(root.right)
+        return output
diff --git a/validate-binary-search-tree/bhyun-kim.py b/validate-binary-search-tree/bhyun-kim.py
index dddf7a59d..b84c1a594 100644
--- a/validate-binary-search-tree/bhyun-kim.py
+++ b/validate-binary-search-tree/bhyun-kim.py
@@ -43,15 +43,12 @@ def isValidBST(self, root: Optional[TreeNode]) -> bool:
         return self.isValidSubTree(root, maximum, minimum)
 
     def isValidSubTree(self, root, maximum, minimum):
-
         if root is None:
             return True
 
-        if not minimum < root.val < maximum: 
-            return False 
-        
+        if not minimum < root.val < maximum:
+            return False
+
         return self.isValidSubTree(
             root.left, root.val, minimum
-            ) and self.isValidSubTree(
-            root.right, maximum, root.val 
-            )
+        ) and self.isValidSubTree(root.right, maximum, root.val)
diff --git a/word-search/bhyun-kim.py b/word-search/bhyun-kim.py
new file mode 100644
index 000000000..ab5208f2a
--- /dev/null
+++ b/word-search/bhyun-kim.py
@@ -0,0 +1,90 @@
+"""
+79. Word Search
+https://leetcode.com/problems/word-search/
+
+Solution:
+    To solve this problem, we think of the board as a graph. 
+    We can use a depth-first search (DFS) to explore all possible paths starting from each cell.
+    We can create a helper function that takes the current cell and the index of the current character in the word.
+
+    - We can count the frequency of each character in the board.
+    - We can check if all characters in the word exist in the board.
+    - If we cannot find all characters in the board, we return False.
+
+    - We can create a helper function that takes the current coordinates and the index in the word.
+    - If the index is equal to the length of the word, 
+        it means we have found all characters in the word, so we return True.
+    - If the coordinates are out of bounds or the current character does not match,
+        we return False.
+    - We mark the current cell as visited and explore all possible directions.
+    - If any direction returns True, we return True.
+    - We unmark the current cell and return False.
+
+    - We can find all indices of the first character in the word.
+    - We can start the DFS from each index and return True if any DFS returns True.
+    - If no DFS returns True, we return False.
+
+Time complexity: O(m*n*4^l)
+    - m and n are the dimensions of the board.
+    - l is the length of the word.
+    - We explore 4 directions at each cell, and the maximum depth is the length of the word.
+
+Space complexity: O(l)
+    - The recursive call stack has a maximum depth of the length of the word. 
+    
+"""
+
+
+from typing import List
+
+
+class Solution:
+    def exist(self, board: List[List[str]], word: str) -> bool:
+        if not board or not word:
+            return False
+
+        m, n = len(board), len(board[0])
+
+        # Step 1: Check if all characters in the word exist in the board
+        char_count = {}
+        for row in board:
+            for char in row:
+                if char in char_count:
+                    char_count[char] += 1
+                else:
+                    char_count[char] = 1
+
+        for char in word:
+            if char not in char_count or char_count[char] == 0:
+                return False
+            char_count[char] -= 1
+
+        # Helper function to check if the word can be found starting from (i, j)
+        def dfs(i, j, word_index):
+            if word_index == len(word):
+                return True
+
+            if i < 0 or i >= m or j < 0 or j >= n or board[i][j] != word[word_index]:
+                return False
+
+            temp = board[i][j]
+            board[i][j] = "#"  # mark as visited
+
+            # Explore all possible directions
+            found = (
+                dfs(i + 1, j, word_index + 1)
+                or dfs(i - 1, j, word_index + 1)
+                or dfs(i, j + 1, word_index + 1)
+                or dfs(i, j - 1, word_index + 1)
+            )
+
+            board[i][j] = temp  # unmark
+            return found
+
+        # Step 2: Find all indices of the first character in the word
+        for i in range(m):
+            for j in range(n):
+                if board[i][j] == word[0] and dfs(i, j, 0):
+                    return True
+
+        return False