BT_Inorder_It02.cpp

class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        if(!root) return res;
        stack<TreeNode*> st;
        TreeNode* pre = nullptr, *cur = root;
        while(cur || !st.empty()){
            while(cur){
                st.push(cur);
                cur = cur->left;
            }
            cur = st.top();
            // pop immediately to avoid repetitively visit
            st.pop(); 
            res.push_back(cur->val);
            if(cur->right && cur->right != pre){
                cur = cur->right;
                //We need to continue for the next loop in order
                //to get the most left nodes to stack
                continue;
            }
            pre = cur;
            cur = nullptr;
        }
        return res;
    }
};