diff --git a/arai60/minimum-depth-of-binary-tree/README.md b/arai60/minimum-depth-of-binary-tree/README.md
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+## 考察
+- 初見の問題
+- 昨日の問題が maximum depth だったのに対して minimum depth
+- 方針
+    - DFS
+        - 帰りがけ
+            - left subtreeとright subtreeのdepthのminを取って1を足して返却
+            - 再帰の深さについて考える必要
+            - 最大10^5のため、デフォルトのPythonの環境では厳しい(一応LeetCode上ならOK)
+            - 帰りがけDFSを非再帰で書くのはつらい
+        - 行きがけ
+            - 全探索しながらleafのときに答えを更新
+    - BFS
+        - level順に見ていくので、leafが見つかったら答えをreturnしてOK
+- 上記を考慮して、Step1はBFSでやる
+- あとは実装
+
+## Step1
+- BFSで実装
+- nodeがNoneのときにはキューに入れない実装もできるが、複雑度(if文)が2つ増えるためこちらの書き方を選択
+- time: O(n), space: O(n)
+
+## Step2
+- 帰りがけDFSと行きがけDFSでも書いてみる
+    - `step2_postorder_dfs.py`
+    - `step2_preorder_dfs.py`
+- 実装のしやすさでは再帰DFS
+- 他の人のPRを検索してみる
+    - BFSはキューを2つ作る書き方も主流みたい
+        - https://github.com/fhiyo/leetcode/pull/24#discussion_r1648682841
+        - この書き方はtorusさんのword ladderでのbidirectional bfsの解法のところでも見た
+        - 前から順番に取り出せればいいのでキューである必要はなさそう
+    - float('inf')について
+        - https://github.com/fhiyo/leetcode/pull/24#discussion_r1648673169
+    - is と == の違い
+        - https://github.com/sakupan102/arai60-practice/pull/23#discussion_r1591177498
+
+## Step3
+- 1回目: 1m16s
+- 2回目: 1m03s
+- 3回目: 1m01s
diff --git a/arai60/minimum-depth-of-binary-tree/step1.py b/arai60/minimum-depth-of-binary-tree/step1.py
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+++ b/arai60/minimum-depth-of-binary-tree/step1.py
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+# 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 minDepth(self, root: Optional[TreeNode]) -> int:
+        nodes_to_visit = deque([(root, 1)])
+        while nodes_to_visit:
+            node, depth = nodes_to_visit.popleft()
+            if not node: continue
+            if not node.left and not node.right:
+                return depth
+            nodes_to_visit.append((node.left, depth + 1))
+            nodes_to_visit.append((node.right, depth + 1))
+
+        return 0
diff --git a/arai60/minimum-depth-of-binary-tree/step2_postorder_dfs.py b/arai60/minimum-depth-of-binary-tree/step2_postorder_dfs.py
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+++ b/arai60/minimum-depth-of-binary-tree/step2_postorder_dfs.py
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+# 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 minDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+        if not root.left and not root.right:
+            return 1
+
+        min_depth = math.inf
+        if root.left:
+            min_depth = min(min_depth, self.minDepth(root.left))
+        if root.right:
+            min_depth = min(min_depth, self.minDepth(root.right))
+
+        return min_depth + 1
diff --git a/arai60/minimum-depth-of-binary-tree/step2_preorder_dfs.py b/arai60/minimum-depth-of-binary-tree/step2_preorder_dfs.py
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+++ b/arai60/minimum-depth-of-binary-tree/step2_preorder_dfs.py
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+# 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 minDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+
+        min_depth = math.inf
+        nodes_to_visit = [(root, 1)]
+        while nodes_to_visit:
+            node, depth = nodes_to_visit.pop()
+            if not node.left and not node.right:
+                min_depth = min(min_depth, depth)
+
+            if node.left:
+                nodes_to_visit.append((node.left, depth + 1))
+            if node.right:
+                nodes_to_visit.append((node.right, depth + 1))
+
+        return min_depth
diff --git a/arai60/minimum-depth-of-binary-tree/step3.py b/arai60/minimum-depth-of-binary-tree/step3.py
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+++ b/arai60/minimum-depth-of-binary-tree/step3.py
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+# 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 minDepth(self, root: Optional[TreeNode]) -> int:
+        nodes_to_visit = deque([(root, 1)])
+        while nodes_to_visit:
+            node, depth = nodes_to_visit.popleft()
+            if not node: continue
+            if not node.left and not node.right:
+                return depth
+            nodes_to_visit.append((node.left, depth + 1))
+            nodes_to_visit.append((node.right, depth + 1))
+        return 0