103. Binary Tree Zigzag Level Order Traversal

0103-binary-tree-zigzag-level-order-traversal/memo.md

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+### step1
+
+基本は102. Binary Tree Level Order Traversalと同じで、booleanをもって階層ごとに一つ飛ばしで配列をreverseした。
+
+### step2
+
+変更点なし。
+
+### step3
+
+3回通すまで書き直し。

0103-binary-tree-zigzag-level-order-traversal/step1.cpp

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+/**
+ * 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>> zigzagLevelOrder(TreeNode* root) {
+        vector<vector<int>> result;
+        if (!root) return result;
+
+        queue<TreeNode*> nodes;
+        nodes.push(root);
+
+        bool toggle = false;
+        while (!nodes.empty()) {
+            int size = nodes.size();
+            vector<int> level;
+
+            for (int i = 0; i < size; i++) {
+                TreeNode* node = nodes.front();
+                nodes.pop();
+
+                level.push_back(node->val);
+
+                if (node->left) nodes.push(node->left);
+                if (node->right) nodes.push(node->right);
+            }
+
+            if (toggle) {
+                reverse(level.begin(), level.end());
+            }
+
+            toggle = !toggle;
+
+            result.push_back(level);
+        }
+
+        return result;
+    }
+};

0103-binary-tree-zigzag-level-order-traversal/step2.cpp

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+/**
+ * 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>> zigzagLevelOrder(TreeNode* root) {
+        vector<vector<int>> result;
+        if (!root) {
+            return result;
+        }
+
+        queue<TreeNode*> nodes;
+        nodes.push(root);
+
+        bool toggle = false;
+        while (!nodes.empty()) {
+            int size = nodes.size();
+            vector<int> level;
+
+            for (int i = 0; i < size; i++) {
+                TreeNode* node = nodes.front();
+                nodes.pop();
+
+                level.push_back(node->val);
+                if (node->left) {
+                    nodes.push(node->left);
+                }
+                if (node->right) {
+                    nodes.push(node->right);
+                }
+            }
+            if (toggle) {
+                reverse(level.begin(), level.end());
+            }
+            toggle = !toggle;
+            result.push_back(level);
+        }
+        return result;
+    }
+};

0103-binary-tree-zigzag-level-order-traversal/step3.cpp

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+/**
+ * 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>> zigzagLevelOrder(TreeNode* root) {
+        vector<vector<int>> result;
+        if (!root) {
+            return result;
+        }
+
+        queue<TreeNode*> nodes;
+        nodes.push(root);
+
+        bool toggle = false;
+        while (!nodes.empty()) {
+            int size = nodes.size();
+            vector<int> level;
+
+            for (int i = 0; i < size; i++) {
+                TreeNode* node = nodes.front();
+                nodes.pop();
+
+                level.push_back(node->val);
+                if (node->left) {
+                    nodes.push(node->left);
+                }
+                if (node->right) {
+                    nodes.push(node->right);
+                }
+            }
+            if (toggle) {
+                reverse(level.begin(), level.end());
+            }
+            toggle = !toggle;
+            result.push_back(level);
+        }
+        return result;
+    }
+};