class Solution {

    public:

    vector<int> inorderTraversal(TreeNode* root) {

        vector<int> res;

        if (root == nullptr)

            return res;

        stack<TreeNode*> sk;

        TreeNode* temp = root;

        while (temp) {

            sk.push(temp);

            temp = temp->left;

        }

        while (!sk.empty()) {

            temp = sk.top();

            sk.pop();

            res.push_back(temp->val);

            temp = temp->right;

            while (temp) {

                sk.push(temp);

                temp = temp->left;

            }

        }

        return res;

    }

};

class Solution {

public:

    vector<int> inorderTraversal(TreeNode* root) {

        vector<int> res;

        stack<TreeNode*> stk;

        while (root != nullptr || !stk.empty()) {

            while (root != nullptr) {

                stk.push(root);

                root = root->left;

            }

            root = stk.top();

            stk.pop();

            res.push_back(root->val);

            root = root->right;

        }

        return res;

    }

};