Binary Search Tree Iterator -- leetcode

Question:

Implement an iterator over a binary search tree (BST). Your iterator will be initialized with the root node of a BST.

Calling next() will return the next smallest number in the BST.

Note: next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.

Answer:

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class BSTIterator {
    stack<TreeNode*> stk;
    int index;
   
public:
    BSTIterator(TreeNode *root) {
        index = 0;
        TreeNode *p = root;
       
        while(p){
            stk.push(p);
            p = p->left;
        }
    }

    /** @return whether we have a next smallest number */
    bool hasNext() {
       return !stk.empty();
    }

    /** @return the next smallest number */
    int next() {
        if(!stk.empty()){
            TreeNode* top = stk.top();
            int res = top->val;
            stk.pop();
            index++;
       
            TreeNode* p = top->right;
            while(p){
                stk.push(p);
                p=p->left;
            }
            return res;
        }
    }
};

/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = BSTIterator(root);
 * while (i.hasNext()) cout << i.next();
 */