Given a binary tree, return the inorder traversal of its nodes' values.
For example:Given binary tree[1,null,2,3],

   1
    \
     2
    /
   3

return [1,3,2].
Note: Recursive solution is trivial, could you do it iteratively?

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