617. Merge Two Binary Trees

0617-merge-two-binary-trees/altanative-solution.cpp

@@ -0,0 +1,36 @@
+/**
+ * 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:
+    TreeNode* mergeTrees(TreeNode* root1, TreeNode* root2) {
+        if (!root1 && !root2) {
+            return nullptr;
+        }
+
+        int val1 = root1 ? root1->val : 0;
+        int val2 = root2 ? root2->val : 0;
+
+        TreeNode* node = new TreeNode(val1 + val2);
+
+        node->left = mergeTrees(
+            root1 ? root1->left : nullptr,
+            root2 ? root2->left : nullptr
+        );
+
+        node->right = mergeTrees(
+            root1 ? root1->right : nullptr,
+            root2 ? root2->right : nullptr
+        );
+
+        return node;
+    }
+};

0617-merge-two-binary-trees/memo.md

@@ -0,0 +1,11 @@
+### step1
+
+どう再帰させたらいいかイメージしながら実装しました。
+
+### step2
+
+無駄な部分をChatGPTにスッキリ書き直してもらいました。
+
+### step3
+
+3回通すまで書き直し。

0617-merge-two-binary-trees/step1.cpp

@@ -0,0 +1,30 @@
+/**
+ * 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:
+    TreeNode* mergeTrees(TreeNode* root1, TreeNode* root2) {
+        if (root1 == nullptr && root2 == nullptr) {
+            return nullptr;
+        } else if (root1 == nullptr) {
+            root1 = root2;
+            return root1;
+        } else if (root2 == nullptr) {
+            root2 = root1;
+            return root2;
+        } else {
+            root1->val += root2->val;
+            root1->left = mergeTrees(root1->left, root2->left);
+            root1->right = mergeTrees(root1->right, root2->right);
+            return root1;
+        }
+    }
+};

0617-merge-two-binary-trees/step2.cpp

@@ -0,0 +1,28 @@
+/**
+ * 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:
+    TreeNode* mergeTrees(TreeNode* root1, TreeNode* root2) {
+        if (!root1) {
+            return root2;
+        }
+        if (!root2) {
+            return root1;
+        }
+
+        root1->val += root2->val;
+        root1->left = mergeTrees(root1->left, root2->left);
+        root1->right = mergeTrees(root1->right, root2->right);
+
+        return root1;
+    }
+};

0617-merge-two-binary-trees/step3.cpp

@@ -0,0 +1,27 @@
+/**
+ * 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:
+    TreeNode* mergeTrees(TreeNode* root1, TreeNode* root2) {
+        if (!root1) {
+            return root2;
+        }
+        if (!root2) {
+            return root1;
+        }
+
+        root1->val += root2->val;
+        root1->left = mergeTrees(root1->left, root2->left);
+        root1->right = mergeTrees(root1->right, root2->right);
+        return root1;
+    }
+};