diff --git a/Week_02/id_38/LeetCode_671_38.py b/Week_02/id_38/LeetCode_671_38.py
new file mode 100644
index 00000000..36688daf
--- /dev/null
+++ b/Week_02/id_38/LeetCode_671_38.py
@@ -0,0 +1,34 @@
+from collections import deque
+
+
+class Solution:
+    def findSecondMinimumValue(self, root):
+        if not root:
+            return -1
+        if not root.left or not root.right:
+            return -1
+        minimum_value = None
+        queue = deque()
+        queue.append(root)
+        while queue:
+            for _ in range(len(queue)):
+                node = queue.popleft()
+                if node.left:
+                    if not minimum_value:
+                        queue.append(node.left)
+                    elif node.left.val <= root.val + 1:
+                        queue.append(node.left)
+                if node.right:
+                    if not minimum_value:
+                        queue.append(node.right)
+                    if node.right.val <= root.val + 1:
+                        queue.append(node.right)
+                if node.val - 1 == root.val:
+                    return node.val
+                if not minimum_value and root.val < node.val:
+                    minimum_value = node.val
+                elif minimum_value and node.val != root.val:
+                    minimum_value = min(minimum_value, node.val)
+        if not minimum_value:
+            return -1
+        return minimum_value
diff --git a/Week_02/id_38/LeetCode_783_38.py b/Week_02/id_38/LeetCode_783_38.py
new file mode 100644
index 00000000..d0c5380a
--- /dev/null
+++ b/Week_02/id_38/LeetCode_783_38.py
@@ -0,0 +1,38 @@
+from collections import deque
+
+
+class Solution:
+    def minDiffInBST(self, root):
+        if not root:
+            return None
+        if not root.left and not root.right:
+            return None
+        stack = deque()
+        visited = []
+        stack.append(root)
+        while stack:
+            node = stack.pop()
+            if node in visited:
+                continue
+            if node.right and node.right not in visited:
+                stack.append(node.right)
+            if not node.left or node.left in visited:
+                visited.append(node)
+            else:
+                stack.append(node)
+            if node.left and node.left not in visited:
+                stack.append(node.left)
+        last_node = None
+        minimum_value = None
+        while visited:
+            current_node = visited.pop()
+            if last_node and last_node.val + 1 == current_node.val:
+                return 1
+            if last_node:
+                space_value = abs(current_node.val - last_node.val)
+            else:
+                space_value = current_node.val
+            last_node = current_node
+            if not minimum_value or (minimum_value and minimum_value > space_value):
+                minimum_value = space_value
+        return minimum_value
diff --git a/Week_02/id_38/NOTE.md b/Week_02/id_38/NOTE.md
index c684e62f..10ad8345 100644
--- a/Week_02/id_38/NOTE.md
+++ b/Week_02/id_38/NOTE.md
@@ -1 +1,57 @@
-# 学习笔记
\ No newline at end of file
+# 学习笔记
+## 二叉查找树（Binary Search Tree）
+二叉查找树有以下特点：
+* 根节点的值大于左子树的值，小于等于右子树的值。
+* 使用二叉树中序遍历的方式可以得到一个有序的数据集。
+* 
+## 二叉树（Binary Tree）
+
+二叉树的遍历：前序遍历，中序遍历，后序遍历，以及广度优先遍历（DFS）和深度优先遍历（BFS）。
+
+前序遍历，中序遍历，后序遍历的实现使用递归的方式很简单。如果用递归可以实现，那么也可以使用迭代的方式实现。
+
+以下代码是用栈实现二叉树非递归的中序遍历。
+
+```python
+def in_order(self, root):
+    stack = deque()
+    visited = []
+    stack.append(root)
+    while stack:
+        node = stack.pop()
+        if node in visited:
+            continue
+        if node.right and node.right not in visited:
+            stack.append(node.right)
+        if not node.left or node.left in visited:
+            visited.append(node)
+        else:
+            stack.append(node)
+        if node.left and node.left not in visited:
+            stack.append(node.left)
+
+    for item in visited:
+        print(item.val)
+```
+
+二叉树的按层遍历（DFS）算法代码实现如下：
+
+```python
+def dfs(self, root):
+    if not root:
+        return
+    depth = 1
+    queue = deque()
+    queue.append(root)
+    while queue:
+        for _ in range(len(queue)):
+            node = queue.popleft()
+            print(node.val)
+            if node.left:
+                queue.append(node.left)
+            if node.right:
+                queue.append(node.right)
+        print("*******************")
+        depth += 1
+```
+
diff --git a/Week_03/id_38/LeetCode_104_38.py b/Week_03/id_38/LeetCode_104_38.py
new file mode 100644
index 00000000..f7ffd468
--- /dev/null
+++ b/Week_03/id_38/LeetCode_104_38.py
@@ -0,0 +1,118 @@
+class Solution:
+
+    def maxDepth(self, root):
+        if not root:
+            return 0
+        return self.get_tree_max_depth(root, 0)
+
+    def get_tree_max_depth(self, root, depth):
+        if not root:
+            return depth
+        return max(self.get_tree_max_depth(root.left, depth), self.get_tree_max_depth(root.right, depth)) + 1
+
+
+class Solution:
+
+    def maxDepth(self, root):
+        if not root:
+            return 0
+        return self.get_tree_max_depth(root, 0)
+
+    def get_tree_max_depth(self, root, depth):
+        if not root:
+            return depth
+        return max(self.get_tree_max_depth(root.left, depth), self.get_tree_max_depth(root.right, depth)) + 1
+
+
+class Solution:
+
+    def maxDepth(self, root):
+        if not root:
+            return 0
+        return self.get_tree_max_depth(root, 0)
+
+    def get_tree_max_depth(self, root, depth):
+        if not root:
+            return depth
+        return max(self.get_tree_max_depth(root.left, depth), self.get_tree_max_depth(root.right, depth)) + 1
+
+
+class Solution:
+
+    def maxDepth(self, root):
+        if not root:
+            return 0
+        return self.get_tree_max_depth(root, 0)
+
+    def get_tree_max_depth(self, root, depth):
+        if not root:
+            return depth
+        return max(self.get_tree_max_depth(root.left, depth), self.get_tree_max_depth(root.right, depth)) + 1
+
+
+class Solution:
+
+    def maxDepth(self, root):
+        if not root:
+            return 0
+        return self.get_tree_max_depth(root, 0)
+
+    def get_tree_max_depth(self, root, depth):
+        if not root:
+            return depth
+        return max(self.get_tree_max_depth(root.left, depth), self.get_tree_max_depth(root.right, depth)) + 1
+
+
+class Solution:
+
+    def maxDepth(self, root):
+        if not root:
+            return 0
+        return self.get_tree_max_depth(root, 0)
+
+    def get_tree_max_depth(self, root, depth):
+        if not root:
+            return depth
+        return max(self.get_tree_max_depth(root.left, depth), self.get_tree_max_depth(root.right, depth)) + 1
+
+
+class Solution:
+
+    def maxDepth(self, root):
+        if not root:
+            return 0
+        return self.get_tree_max_depth(root, 0)
+
+    def get_tree_max_depth(self, root, depth):
+        if not root:
+            return depth
+        return max(self.get_tree_max_depth(root.left, depth), self.get_tree_max_depth(root.right, depth)) + 1
+
+
+class Solution:
+
+    def maxDepth(self, root):
+        if not root:
+            return 0
+        return self.get_tree_max_depth(root, 0)
+
+    def get_tree_max_depth(self, root, depth):
+        if not root:
+            return depth
+        return max(self.get_tree_max_depth(root.left, depth), self.get_tree_max_depth(root.right, depth)) + 1
+
+
+class Solution:
+
+    def maxDepth(self, root):
+        if not root:
+            return 0
+        return self.get_tree_max_depth(root, 0)
+
+    def get_tree_max_depth(self, root, depth):
+        if not root:
+            return depth
+        return max(self.get_tree_max_depth(root.left, depth), self.get_tree_max_depth(root.right, depth)) + 1
+
+
+
diff --git a/Week_03/id_38/LeetCode_429_38.py b/Week_03/id_38/LeetCode_429_38.py
new file mode 100644
index 00000000..9e0d35e9
--- /dev/null
+++ b/Week_03/id_38/LeetCode_429_38.py
@@ -0,0 +1,21 @@
+from collections import deque
+
+
+class Solution:
+    def levelOrder(self, root):
+        result = []
+        if not root:
+            return result
+        queue = deque()
+        queue.append(root)
+        while queue:
+            items = []
+            for _ in range(len(queue)):
+                node = queue.popleft()
+                items.append(node.val)
+                for item in node.children:
+                    queue.append(item)
+            result.append(items)
+        return result
+
+
diff --git a/Week_03/id_38/LeetCode_703_38.py b/Week_03/id_38/LeetCode_703_38.py
new file mode 100644
index 00000000..7505941f
--- /dev/null
+++ b/Week_03/id_38/LeetCode_703_38.py
@@ -0,0 +1,19 @@
+import heapq
+
+
+class KthLargest:
+    def __init__(self, k, nums):
+        self.nums = nums
+        self.k = k
+        heapq.heapify(self.nums)
+        while len(self.nums) > k:
+            heapq.heappop(self.nums)
+
+    def add(self, val):
+        if len(self.nums) < self.k:
+            heapq.heappush(self.nums, val)
+        elif val > self.nums[0]:
+            heapq.heapreplace(self.nums, val)
+        return self.nums[0]
+
+
diff --git a/Week_03/id_38/LeetCode_997_38.py b/Week_03/id_38/LeetCode_997_38.py
new file mode 100644
index 00000000..06c024d7
--- /dev/null
+++ b/Week_03/id_38/LeetCode_997_38.py
@@ -0,0 +1,25 @@
+class Solution:
+    def findJudge(self, n, trusts):
+        if n == 1 and len(trusts) == 0:
+            return 1
+        followers = [None] * (n + 1)
+        followings = [None] * (n + 1)
+        for trust in trusts:
+            person = trust[0]
+            following = trust[1]
+            if not followings[person]:
+                followings[person] = []
+            followings[person].append(following)
+            if not followers[following]:
+                followers[following] = []
+            followers[following].append(person)
+        for i in range(1, n + 1):
+            follower = followers[i]
+            following = followings[i]
+            if following or not follower:
+                continue
+            if len(follower) == n - 1:
+                return i
+        return -1
+
+
diff --git a/Week_04/id_38/LeetCode_169_38.py b/Week_04/id_38/LeetCode_169_38.py
new file mode 100644
index 00000000..4f3d143b
--- /dev/null
+++ b/Week_04/id_38/LeetCode_169_38.py
@@ -0,0 +1,6 @@
+class Solution:
+    def majorityElement(self, nums):
+        nums = sorted(nums)
+        return nums[len(nums) // 2]
+
+
diff --git a/Week_04/id_38/LeetCode_702_38.py b/Week_04/id_38/LeetCode_702_38.py
new file mode 100644
index 00000000..6926fb4c
--- /dev/null
+++ b/Week_04/id_38/LeetCode_702_38.py
@@ -0,0 +1,29 @@
+class TrieNode:
+    def __init__(self, data):
+        self.data = data
+        self.isEnding = False
+        self.children = [None] * 26
+
+class Solution:
+    def longestWord(self, words):
+        words = sorted(words)
+        longest_word = None
+        root = TrieNode("/")
+        for i in range(len(words)):
+            word = words[i]
+            if not longest_word:
+                longest_word = word
+            p = root
+            length_of_word = len(word)
+            word_is_valid = True
+            for j in range(length_of_word):
+                index = ord(word[j]) - ord("a")
+                if not p.children[index] and j == length_of_word - 1:
+                    p.children[index] = TrieNode(word[j])
+                elif not p.children[index]:
+                    word_is_valid = False
+                    break
+                p = p.children[index]
+            if len(word) > len(longest_word) and word > longest_word and word_is_valid:
+                longest_word = word
+        return longest_word