Binary Search Tree Iterator / Implementing Forward Iterator in BST

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */

/*
Logic:
We will use a stack and push the root node and it's all left elements in stack in constructor.
next function want the next element of inorder so return the top element of stack and push the
right element of top element of stack and push all the left elements

hasNext : it return true if there is next element in inorder.
*/


class BSTIterator {
    stack<TreeNode*> st;
public:
    BSTIterator(TreeNode* root) {
       while(root){
           st.push(root);
           root=root->left;
       }
    }

    int next() {
        TreeNode *temp=st.top();
        st.pop();
        int res=temp->val;
        if(temp->right){st.push(temp->right);temp=temp->right;
            temp=temp->left;
            while(temp){
                st.push(temp);
                temp=temp->left;
            }
        }
        return res;
    }

    bool hasNext() {
        if(st.size()) return true;
        return false;
    }
};