/**
knakul853
 */
class Solution {
public:
    vector<int>ans;
    vector<int> inorderTraversal(TreeNode* root) {

       stack<TreeNode*> st;
        vector<int>ans;
        while(!st.empty() || root)
        {
           while( root != NULL)
           {
               st.push(root);
               root = root->left;
           }
            root = st.top();st.pop();
            ans.push_back(root->val);
            root = root->right;
         }
        return ans;
    }
};

/**

class Solution {
public:
    vector<int>ans;
    vector<int> inorderTraversal(TreeNode* root) {

        if( !root ) return {};

        inorderTraversal(root->left);
        ans.push_back(root->val);
        inorderTraversal(root->right);
        return ans;

    }
};

**/