From 340ef010f8e2a78173d6032d5c0b66c2393fcd4c Mon Sep 17 00:00:00 2001
From: Hiro Funatsuka <hiro.funatsuka@gmail.com>
Date: Sat, 25 Apr 2026 22:20:33 +0900
Subject: [PATCH] Solve minimum depth of binary tree

---
 README.md |  2 +-
 step1.md  | 56 +++++++++++++++++++++++++++++++++++++++++++++++++++++--
 step2.md  | 22 ++++++++++++++++++++++
 step3.md  | 27 ++++++++++++++++++++++++---
 4 files changed, 101 insertions(+), 6 deletions(-)

diff --git a/README.md b/README.md
index 0ee68b0..0b0055c 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 # LeetCode
 
-## 問題: [Link Text](URL)
+## 問題: [111. Minimum Depth of Binary Tree](https://leetcode.com/problems/minimum-depth-of-binary-tree/description/)
 
 ## 前提
 
diff --git a/step1.md b/step1.md
index 54ebbc1..3d5dd9f 100644
--- a/step1.md
+++ b/step1.md
@@ -1,7 +1,59 @@
 # Step 1
 
+## Recursive DFS (失敗)
+
+- nodeがNoneだったら0を返す
+- 左右最小の深さに1を足して返す
+
+```python
+# 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 root is None:
+            return 0
+        return 1 + min(self.minDepth(root.left), self.minDepth(root.right))
+```
+
+- 葉が片方にしかないとき、rootに0が返ってくる
+
+時間計算量: O(n)
+
+空間計算量: O(n)
+
+## Iterative DFS
+
+- nodeはNoneではないが、left, rightがNoneの場合に最小の深さを更新する
+
 ```python
+# 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 root is None:
+            return 0
+
+        res = 10 ** 5
+        stack = [[root, 1]]
+        while stack:
+            node, depth = stack.pop()
+            if node:
+                if node.left is None and node.right is None:
+                    res = min(res, depth)
+                else:
+                    stack.append([node.left, depth + 1])
+                    stack.append([node.right, depth + 1])
+        return res
 ```
-時間計算量:
 
-空間計算量:
+時間計算量: O(n)
+
+空間計算量: O(n)
diff --git a/step2.md b/step2.md
index 6ec3360..ab54427 100644
--- a/step2.md
+++ b/step2.md
@@ -1,4 +1,26 @@
 # Step 2
 
 ```python
+# 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 root is None:
+            return 0
+
+        min_depth = float("inf")
+        stack = [(root, 1)]
+        while stack:
+            node, depth = stack.pop()
+            if node:
+                if node.left is None and node.right is None:
+                    min_depth = min(res, depth)
+                else:
+                    stack.append((node.left, depth + 1))
+                    stack.append((node.right, depth + 1))
+        return min_depth
 ```
diff --git a/step3.md b/step3.md
index 522daa6..9849ce4 100644
--- a/step3.md
+++ b/step3.md
@@ -1,9 +1,30 @@
 # Step 3
 
 ```python
+# 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 root is None:
+            return 0
+        min_depth = float("inf")
+        stack = [(root, 1)]
+        while stack:
+            node, depth = stack.pop()
+            if node:
+                if node.left is None and node.right is None:
+                    min_depth = min(min_depth, depth)
+                else:
+                    stack.append((node.left, depth + 1))
+                    stack.append((node.right, depth + 1))
+        return min_depth
 ```
-1回目: 分　秒
+1回目: 2分　11秒
 
-2回目: 分　秒
+2回目: 2分　6秒
 
-3回目: 分　秒
+3回目: 2分　8秒