diff --git a/memo.md b/memo.md
index 4bd0397..d7b739a 100644
--- a/memo.md
+++ b/memo.md
@@ -1 +1,196 @@
 # Step1
+
+## アプローチ
+
+* 最も深い場所を知りたい
+    * BFSかDFS
+    * これは全てのノードを訪問する必要があるのでO(N)
+
+
+## Code1-1 (DFS) - solved 3:52
+
+```python
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if root is None:
+            return 0
+
+        node = root
+        frontier = [(node, 1)]
+        maximum_depth = 0
+        while frontier:
+            node, depth = frontier.pop()
+            maximum_depth = max(maximum_depth, depth)
+            if node.left is not None:
+                frontier.append((node.left, depth + 1))
+            if node.right is not None:
+                frontier.append((node.right, depth + 1))
+        
+        return maximum_depth
+            
+```
+
+## Code1-2 (Recursion) - solved 0:46
+
+```python
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if root is None:
+            return 0
+        
+        left_max_depth = self.maxDepth(root.left)
+        right_max_depth = self.maxDepth(root.right)
+        return max(left_max_depth, right_max_depth) + 1
+
+```
+
+## Code1-3 (BFS) - solved 2:12
+
+```python
+from collections import deque
+
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if root is None:
+            return 0
+
+        frontier = deque()
+        frontier.append(root)
+        depth = 0
+        while frontier:
+            num_cur_frontiers = len(frontier)
+            depth += 1
+            for _ in range(num_cur_frontiers):
+                node = frontier.popleft()
+                if node.left is not None:
+                    frontier.append(node.left)
+                if node.right is not None:
+                    frontier.append(node.right)
+        
+        return depth
+
+```
+
+# 他の人のコードや調べたこと
+
+* 1 - https://github.com/n6o/leetcode_arai60/pull/20/changes
+    * 思いついた解法３種類全部一緒だった
+    * 実装もほぼ一緒
+* 2 - https://github.com/TakayaShirai/leetcode_practice/pull/21
+    * 同様に解法３種類やっている
+* 3 - https://github.com/Hiroto-Iizuka/coding_practice/pull/21
+    * Pythonだと再帰に忌避感を持っていた方がいいかも
+        * > 個人的にはPythonで再帰を書くことにかなり抵抗があるので、iterativeでもこの程度のコード量で済むならiterativeで書きたいと思いました。
+* 4 - https://github.com/xbam326/leetcode/pull/23
+* 5 - https://github.com/dxxsxsxkx/leetcode/pull/21
+
+
+# Step2
+
+## Code2-1 (DFDS)
+
+```python
+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 maxDepth(self, root: Optional[TreeNode]) -> int:
+        if root is None:
+            return 0
+
+        node = root
+        frontier = [(node, 1)]
+        maximum_depth = 0
+        while frontier:
+            node, depth = frontier.pop()
+            if node is None:
+                continue
+            maximum_depth = max(maximum_depth, depth)
+            frontier.append((node.left, depth + 1))
+            frontier.append((node.right, depth + 1))
+        
+        return maximum_depth
+
+```
+
+## Code2-2 (Recursion)
+
+```python
+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 maxDepth(self, root: Optional[TreeNode]) -> int:
+        if root is None:
+            return 0
+        
+        return max(self.maxDepth(root.left), self.maxDepth(root.right)) + 1
+
+```
+
+## Code2-3(BFS)
+
+```python
+from typing import Optional
+from collections import deque
+
+
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if root is None:
+            return 0
+
+        frontier = deque()
+        frontier.append(root)
+        depth = 0
+        while frontier:
+            num_cur_frontiers = len(frontier)
+            depth += 1
+            for _ in range(num_cur_frontiers):
+                node = frontier.popleft()
+                if node is None:
+                    continue
+                frontier.append(node.left)
+                frontier.append(node.right)
+        
+        return depth
+
+```
+
+# Step3
\ No newline at end of file
diff --git a/step1-1_DFS.py b/step1-1_DFS.py
new file mode 100644
index 0000000..164c2da
--- /dev/null
+++ b/step1-1_DFS.py
@@ -0,0 +1,24 @@
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if root is None:
+            return 0
+
+        node = root
+        frontier = [(node, 1)]
+        maximum_depth = 0
+        while frontier:
+            node, depth = frontier.pop()
+            maximum_depth = max(maximum_depth, depth)
+            if node.left is not None:
+                frontier.append((node.left, depth + 1))
+            if node.right is not None:
+                frontier.append((node.right, depth + 1))
+        
+        return maximum_depth
+            
diff --git a/step1-2_recursion.py b/step1-2_recursion.py
new file mode 100644
index 0000000..da41867
--- /dev/null
+++ b/step1-2_recursion.py
@@ -0,0 +1,14 @@
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if root is None:
+            return 0
+        
+        left_max_depth = self.maxDepth(root.left)
+        right_max_depth = self.maxDepth(root.right)
+        return max(left_max_depth, right_max_depth) + 1
\ No newline at end of file
diff --git a/step1-3_BFS.py b/step1-3_BFS.py
new file mode 100644
index 0000000..2ec272a
--- /dev/null
+++ b/step1-3_BFS.py
@@ -0,0 +1,27 @@
+from collections import deque
+
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if root is None:
+            return 0
+
+        frontier = deque()
+        frontier.append(root)
+        depth = 0
+        while frontier:
+            num_cur_frontiers = len(frontier)
+            depth += 1
+            for _ in range(num_cur_frontiers):
+                node = frontier.popleft()
+                if node.left is not None:
+                    frontier.append(node.left)
+                if node.right is not None:
+                    frontier.append(node.right)
+        
+        return depth
\ No newline at end of file
diff --git a/step2-1_DFS.py b/step2-1_DFS.py
new file mode 100644
index 0000000..58ee6c9
--- /dev/null
+++ b/step2-1_DFS.py
@@ -0,0 +1,26 @@
+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 maxDepth(self, root: Optional[TreeNode]) -> int:
+        if root is None:
+            return 0
+
+        node = root
+        frontier = [(node, 1)]
+        maximum_depth = 0
+        while frontier:
+            node, depth = frontier.pop()
+            if node is None:
+                continue
+            maximum_depth = max(maximum_depth, depth)
+            frontier.append((node.left, depth + 1))
+            frontier.append((node.right, depth + 1))
+        
+        return maximum_depth
\ No newline at end of file
diff --git a/step2-2_recursion.py b/step2-2_recursion.py
new file mode 100644
index 0000000..573c0ee
--- /dev/null
+++ b/step2-2_recursion.py
@@ -0,0 +1,15 @@
+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 maxDepth(self, root: Optional[TreeNode]) -> int:
+        if root is None:
+            return 0
+        
+        return max(self.maxDepth(root.left), self.maxDepth(root.right)) + 1
\ No newline at end of file
diff --git a/step2-3_BFS.py b/step2-3_BFS.py
new file mode 100644
index 0000000..5f94159
--- /dev/null
+++ b/step2-3_BFS.py
@@ -0,0 +1,29 @@
+from typing import Optional
+from collections import deque
+
+
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if root is None:
+            return 0
+
+        frontier = deque()
+        frontier.append(root)
+        depth = 0
+        while frontier:
+            num_cur_frontiers = len(frontier)
+            depth += 1
+            for _ in range(num_cur_frontiers):
+                node = frontier.popleft()
+                if node.left is not None:
+                    frontier.append(node.left)
+                if node.right is not None:
+                    frontier.append(node.right)
+        
+        return depth
\ No newline at end of file
diff --git a/step3-1_DFS.py b/step3-1_DFS.py
new file mode 100644
index 0000000..ad67890
--- /dev/null
+++ b/step3-1_DFS.py
@@ -0,0 +1,26 @@
+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 maxDepth(self, root: Optional[TreeNode]) -> int:
+        if root is None:
+            return 0
+        
+        frontier = [(root, 1)]
+        max_depth = 0
+        while frontier:
+            node, depth = frontier.pop()
+            max_depth = max(depth, max_depth)
+            if node.left is not None:
+                frontier.append((node.left, depth + 1))
+            if node.right is not None:
+                frontier.append((node.right, depth + 1))
+        
+        return max_depth
+            
\ No newline at end of file
diff --git a/step3-2_recursion.py b/step3-2_recursion.py
new file mode 100644
index 0000000..573c0ee
--- /dev/null
+++ b/step3-2_recursion.py
@@ -0,0 +1,15 @@
+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 maxDepth(self, root: Optional[TreeNode]) -> int:
+        if root is None:
+            return 0
+        
+        return max(self.maxDepth(root.left), self.maxDepth(root.right)) + 1
\ No newline at end of file
diff --git a/step3-3_BFS.py b/step3-3_BFS.py
new file mode 100644
index 0000000..e9589bd
--- /dev/null
+++ b/step3-3_BFS.py
@@ -0,0 +1,28 @@
+from typing import Optional
+from collections import deque
+
+
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if root is None:
+            return 0
+        
+        frontier = deque()
+        frontier.append(root)
+        depth = 0
+        while frontier:
+            num_cur_frontier = len(frontier)
+            depth += 1
+            for _ in range(num_cur_frontier):
+                node = frontier.popleft()
+                if node.left is not None:
+                    frontier.append(node.left)
+                if node.right is not None:
+                    frontier.append(node.right)
+        return depth
\ No newline at end of file