vault backup: 2022-07-04 15:08:23

Leetcode Binary-Tree-Preorder-Traversal.md

@@ -56,9 +56,45 @@ Preorder, means root is at the "Pre" position, so the order is:
 - Search root node
 - Search left sub-tree
 - Search right sub-tree
+
 The recursion version is easy to implement.
 
 ### Solution
+Recursion, using private funcs:
+```cpp
+/**
+ * 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<int> preorderTraversal(TreeNode *root) {
+    // Use two functions:
+    vector<int> answer;
+    preorder(root, answer);
+    return answer;
+  }
+
+private:
+  void preorder(TreeNode *root, vector<int> &answer) {
+    if (root == nullptr) {
+      return;
+    }
+
+    answer.push_back(root->val);
+    preorder(root->left, answer);
+    preorder(root->right, answer);
+  }
+};
+```
 
 Recursion:
 ```cpp