Given a binary tree, return theinordertraversal of its nodes' values.

Example:

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

Output: [1,3,2]

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

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