From 6a9216da91164299f9652bfb97d93ddd90236f6f Mon Sep 17 00:00:00 2001
From: ryosuketc <43229670+ryosuketc@users.noreply.github.com>
Date: Tue, 13 Jan 2026 05:56:38 +0900
Subject: [PATCH] 102. Binary Tree Level Order Traversal

https://leetcode.com/problems/binary-tree-level-order-traversal/
---
 102_binary_tree_level_order_traversal/memo.md | 21 +++++++++++
 .../step1.cpp                                 | 34 +++++++++++++++++
 .../step2.cpp                                 | 37 +++++++++++++++++++
 .../step3.cpp                                 |  0
 4 files changed, 92 insertions(+)
 create mode 100644 102_binary_tree_level_order_traversal/memo.md
 create mode 100644 102_binary_tree_level_order_traversal/step1.cpp
 create mode 100644 102_binary_tree_level_order_traversal/step2.cpp
 create mode 100644 102_binary_tree_level_order_traversal/step3.cpp

diff --git a/102_binary_tree_level_order_traversal/memo.md b/102_binary_tree_level_order_traversal/memo.md
new file mode 100644
index 0000000..e26e2cf
--- /dev/null
+++ b/102_binary_tree_level_order_traversal/memo.md
@@ -0,0 +1,21 @@
+# 102. Binary Tree Level Order Traversal
+
+https://leetcode.com/problems/binary-tree-level-order-traversal/
+
+## Comments
+
+### step1
+
+*   Arai60 で解いた問題
+*   https://github.com/ryosuketc/leetcode_arai60/pull/26/files
+*   とりあえず各 level ごとに処理する方法。queue を使おうかと思ったが nodes (current level) と next_nodes の入れ替えだけで pop などするわけではなかったので vector のまま書いた
+*   とはいえ久々に解いたので12:00くらいかかっった。ちょっとlevelの区切りをどうやって見つけるんだったか悩んでしまった。
+
+### step2
+
+*   一応 queue を使う方法。
+*   あとは上記自分の解答のように (node, level) の tuple を queue に入れていく方法もあるが、level ごとに処理する今回の方法がよりわかりやすいとは思う。
+
+### step3
+
+*   skip
diff --git a/102_binary_tree_level_order_traversal/step1.cpp b/102_binary_tree_level_order_traversal/step1.cpp
new file mode 100644
index 0000000..68b4e07
--- /dev/null
+++ b/102_binary_tree_level_order_traversal/step1.cpp
@@ -0,0 +1,34 @@
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+class Solution {
+public:
+    vector<vector<int>> levelOrder(TreeNode* root) {
+        if (!root) return {};
+        std::vector<std::vector<int>> result;
+        std::vector<TreeNode*> nodes = {root};  // current-level nodes.
+        std::vector<TreeNode*> next_nodes;  // next-level nodes.
+
+        while (!nodes.empty()) {
+            std::vector<int> current_level_values;
+            for (auto node : nodes) {
+                current_level_values.push_back(node->val);
+                if (node->left) next_nodes.push_back(node->left);
+                if (node->right) next_nodes.push_back(node->right);
+            }
+            result.push_back(current_level_values);
+            nodes = next_nodes;
+            next_nodes.clear();
+        }
+
+        return result;
+    }
+};
diff --git a/102_binary_tree_level_order_traversal/step2.cpp b/102_binary_tree_level_order_traversal/step2.cpp
new file mode 100644
index 0000000..d4ff1de
--- /dev/null
+++ b/102_binary_tree_level_order_traversal/step2.cpp
@@ -0,0 +1,37 @@
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+
+#include <queue>
+
+class Solution {
+public:
+    vector<vector<int>> levelOrder(TreeNode* root) {
+        if (!root) return {};
+        std::vector<std::vector<int>> result;
+        std::queue<TreeNode*> nodes;
+        nodes.push(root);
+
+        while (!nodes.empty()) {
+            int level_size = nodes.size();
+            std::vector<int> current_level_values;
+            for (int i = 0; i < level_size; ++i) {
+                TreeNode* node = nodes.front();
+                nodes.pop();
+                current_level_values.push_back(node->val);
+                if (node->left) nodes.push(node->left);
+                if (node->right) nodes.push(node->right);
+            }
+        result.push_back(current_level_values);
+        }
+        return result;
+    }
+};
diff --git a/102_binary_tree_level_order_traversal/step3.cpp b/102_binary_tree_level_order_traversal/step3.cpp
new file mode 100644
index 0000000..e69de29