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?

　　

/**
 * Definition for binary tree
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<int> res;
        if(root == NULL) return res;
    	
		stack<TreeNode *> mys;
		while(! (mys.empty()) || root!= NULL)
		{
			while(root!= NULL){
				mys.push(root);
				root = root->left;
			}
			
			root = mys.top();
			mys.pop();
			res.push_back(root->val);
			root = root->right;
		}
		
		return res;
    }
};