diff --git a/combination-sum/evan.py b/combination-sum/evan.py
new file mode 100644
index 000000000..97ae7a983
--- /dev/null
+++ b/combination-sum/evan.py
@@ -0,0 +1,42 @@
+from typing import List
+
+
+class Solution:
+    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
+        def backtrack(remain, combination, candidateIndex):
+            if remain == 0:
+                # if we reach the target, add the combination to the result
+                result.append(list(combination))
+                return
+            elif remain < 0:
+                # if we exceed the target, no need to proceed
+                return
+
+            for i in range(candidateIndex, len(candidates)):
+                # add the number to the current combination
+                combination.append(candidates[i])
+                # continue the exploration with the current number
+                backtrack(remain - candidates[i], combination, i)
+                # backtrack by removing the number from the combination
+                combination.pop()
+
+        result = []
+        backtrack(target, [], 0)
+
+        return result
+
+
+# Time Complexity: O(N^(target / min(candidates)))
+# The time complexity depends on:
+# - N: The number of candidates (branching factor at each level).
+# - target: The target sum we need to achieve.
+# - min(candidates): The smallest element in candidates influences the maximum depth of the recursion tree.
+# In the worst case, we branch N times at each level, and the depth can be target / min(candidates).
+# Therefore, the overall time complexity is approximately O(N^(target / min(candidates))).
+
+# Space Complexity: O(target / min(candidates))
+# The space complexity is influenced by the maximum depth of the recursion tree:
+# - target: The target sum we need to achieve.
+# - min(candidates): The smallest element in candidates influences the maximum depth.
+# The recursion stack can go as deep as target / min(candidates), so the space complexity is O(target / min(candidates)).
+# Additionally, storing the results can take significant space, but it depends on the number of valid combinations found.
diff --git a/construct-binary-tree-from-preorder-and-inorder-traversal/evan.py b/construct-binary-tree-from-preorder-and-inorder-traversal/evan.py
new file mode 100644
index 000000000..eb4248a40
--- /dev/null
+++ b/construct-binary-tree-from-preorder-and-inorder-traversal/evan.py
@@ -0,0 +1,37 @@
+from typing import List, Optional
+
+
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+
+class Solution:
+    def buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:
+        if not preorder or not inorder:
+            return None
+
+        root_val = preorder.pop(0)
+
+        root_inorder_index = inorder.index(root_val)
+
+        left_tree_inorder = inorder[:root_inorder_index]
+        right_tree_inorder = inorder[root_inorder_index + 1 :]
+
+        return TreeNode(
+            root_val,
+            self.buildTree(preorder, left_tree_inorder),
+            self.buildTree(preorder, right_tree_inorder),
+        )
+
+
+# Overall time complexity: O(N^2)
+# - Finding the root in the inorder list and splitting it takes O(N) time in each call.
+# - There are N nodes, so this operation is repeated N times.
+
+# Overall space complexity: O(N)
+# - The primary space usage is the recursion stack which goes as deep as the height of the tree.
+# - In the worst case (unbalanced tree), this can be O(N).
+# - In the best case (balanced tree), this is O(log N).
diff --git a/implement-trie-prefix-tree/evan.py b/implement-trie-prefix-tree/evan.py
new file mode 100644
index 000000000..b04078e26
--- /dev/null
+++ b/implement-trie-prefix-tree/evan.py
@@ -0,0 +1,37 @@
+class Trie:
+    def __init__(self):
+        self.children = {}
+        self.is_end_of_word = False
+
+    def insert(self, word: str) -> None:
+        node = self
+
+        for char in word:
+            if char not in node.children:
+                node.children[char] = Trie()
+
+            node = node.children[char]
+
+        node.is_end_of_word = True
+
+    def search(self, word: str) -> bool:
+        node = self
+
+        for char in word:
+            if char not in node.children:
+                return False
+
+            node = node.children[char]
+
+        return node.is_end_of_word
+
+    def startsWith(self, prefix: str) -> bool:
+        node = self
+
+        for char in prefix:
+            if char not in node.children:
+                return False
+
+            node = node.children[char]
+
+        return True
diff --git a/kth-smallest-element-in-a-bst/evan.py b/kth-smallest-element-in-a-bst/evan.py
new file mode 100644
index 000000000..061a5e943
--- /dev/null
+++ b/kth-smallest-element-in-a-bst/evan.py
@@ -0,0 +1,40 @@
+# Definition for a binary tree node.
+from typing import Optional
+
+
+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:
+        result = []
+
+        def inorderTraverse(node: Optional[TreeNode]):
+            if node is None or len(result) >= k:
+                return
+
+            inorderTraverse(node.left)
+
+            if len(result) < k:
+                result.append(node.val)
+
+            inorderTraverse(node.right)
+
+        inorderTraverse(root)
+
+        return result[k - 1]
+
+
+# Time Complexity: O(N)
+# In the worst case, we need to visit all the nodes in the tree.
+# Thus, the time complexity is O(N), where N is the number of nodes in the tree.
+
+# Space Complexity: O(N)
+# The space complexity is determined by the recursion stack and the result list.
+# 1. Recursion stack: In the worst case (unbalanced tree), the recursion stack can go up to N levels deep, so the space complexity is O(N).
+#    In the best case (balanced tree), the recursion stack depth is log(N), so the space complexity is O(log N).
+# 2. Result list: The result list stores up to k elements, so the space complexity is O(k).
diff --git a/word-search/evan.py b/word-search/evan.py
new file mode 100644
index 000000000..6315ff189
--- /dev/null
+++ b/word-search/evan.py
@@ -0,0 +1,40 @@
+from typing import List
+
+
+class Solution:
+    def exist(self, board: List[List[str]], word: str) -> bool:
+        def dfs(row, col, k):
+            if k == len(word):
+                return True
+
+            # out of range
+            if row < 0 or row >= len(board) or col < 0 or col >= len(board[0]):
+                return False
+
+            # char not found
+            if board[row][col] != word[k]:
+                return False
+
+            temp = board[row][col]
+
+            # mark visited char
+            board[row][col] = None
+
+            result = (
+                dfs(row + 1, col, k + 1)  # top
+                or dfs(row - 1, col, k + 1)  # bottom
+                or dfs(row, col + 1, k + 1)  # right
+                or dfs(row, col - 1, k + 1)  # left
+            )
+
+            # restore char
+            board[row][col] = temp
+
+            return result
+
+        for row in range(len(board)):
+            for col in range(len(board[0])):
+                if dfs(row, col, 0):
+                    return True
+
+        return False