Create Minimum_Depth_of_Binary_Tree.md

Minimum_Depth_of_Binary_Tree.md

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+一回目。答えを見た。
+
+```py
+class Solution:
+    def minDepth(self, root: Optional[TreeNode]) -> int:
+        if root is None:
+            return 0
+
+        queue = [(root, 1)]
+        while queue:
+            node, depth = queue.pop(0)
+            if node.left is None and node.right is None:
+                return depth
+            if node.left:
+                queue.append((node.left, depth + 1))
+            if node.right:
+                queue.append((node.right, depth + 1))
+```
+
+2回目と3回目。子ノードが None かどうかの判定は（1つ目と2, 3個目で）そろえたほうがよいと判断した。
+
+```py
+class Solution:
+    def minDepth(self, root: Optional[TreeNode]) -> int:
+        if root is None:
+            return 0
+
+        queue = [(root, 1)]
+        while queue:
+            node, depth = queue.pop(0)
+            if node.left is None and node.right is None:
+                return depth
+            if node.left is not None:
+                queue.append((node.left, depth + 1))
+            if node.right is not None:
+                queue.append((node.right, depth + 1))
+```