leetcode[94]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? > read more on how binary tree is serialized on OJ.

/**
 * 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) 
{
    vector<int> res;
    if(root==NULL)return res;
    TreeNode *p=root;
    stack<TreeNode *> sta;
    while(p!=NULL||!sta.empty())
    {
        while(p!=NULL)
        {
            sta.push(p);
            p=p->left;
        }
        if(!sta.empty())
        {
            p=sta.top();
            sta.pop();
            res.push_back(p->val);
            p=p->right;
        }
    }
    return res;
}
/*
void inorder(TreeNode *root, vector<int> &res)
{
    if(root==NULL)return;
    inorder(root->left,res);
    res.push_back(root->val);
    inorder(root->right,res);
    return;
}
    vector<int> inorderTraversal(TreeNode *root) {
        vector<int> res;
        inorder(root,res);
        return res;
    }
*/
};