BT_Inorder_It01.cpp

class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        stack<TreeNode*> st;
        if(root == nullptr) return res;
        //Maintain a variable for current value. For inorder traversal, 
        //we need to push left nodes to our stack, then add the value to res.
        TreeNode* cur = root;
        while(cur || !st.empty()){
            if(cur!= nullptr){
                st.push(cur);
                cur = cur->left;
            }
            else{
                //We need to update cur. because currentlt it's nullptr
                cur = st.top();
                res.push_back(cur->val);
                st.pop();
                cur = cur->right;
            }
        }
        return res;
    }
};