https://leetcode.com/problems/binary-tree-inorder-traversal/

Given a binary tree, return the inorder traversal of its nodes' values.

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,3,2]

程式碼：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> ans;
        inorder(ans, root);
        return ans;
    }
    void inorder(vector<int>& ans, TreeNode* root) {
        if(root == NULL) return ;
        inorder(ans, root -> left);
        ans.push_back(root -> val);
        inorder(ans, root -> right);
    }
};

　　二叉樹 get 第一個 Medium 中序遍歷