Leetcode Binary Tree Inorder Traversal

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

For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

 

return [1,3,2].

Note: Recursive solution is trivial, could you do it iteratively?

confused what "{1,#,2,3}" means? 

遞迴解決方案

class Solution {
public:
    vector<int> res;
    
    void inorder(TreeNode* root){
        if(root == NULL) return;
        inorder(root->left);
        res.push_back(root->val);
        inorder(root->right);
    }
    
    vector<int> inorderTraversal(TreeNode *root) {
        inorder(root);
        return res;
    }
};

 非遞迴解決方案

class Solution {
public:
    vector<int> inorderTraversal(TreeNode *root) {
        vector<int> res;
        if(root == NULL) return res;
        stack<TreeNode *> nodeStack;
        TreeNode *current = root;
        while(!nodeStack.empty() || current ){
            if(current != NULL){
                nodeStack.push(current);
                current = current->left;
            }else{
                current = nodeStack.top();nodeStack.pop();
                res.push_back(current->val);
                current = current->right;
            }
        }
        return res;
    }
};