Binary Search Tree Iterator @LeetCode

PROBLEM :

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.

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SIMPLE c++ IMPLEMENTATION :
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/**
 * 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 *> sta ;
public:
    BSTIterator(TreeNode *root) {
        pushLeft(root) ;
    }

    /** @return whether we have a next smallest number */
    bool hasNext() {
        if(sta.empty())
            return false ;
        return true ;
    }

    /** @return the next smallest number */
    int next() {
        TreeNode *curr=sta.top() ;
        sta.pop() ;
        if(curr->right)
            pushLeft(curr->right) ;
        return curr->val ;
    }
   
    void pushLeft(TreeNode *root){
        TreeNode *ptr=root ;
       
        while(ptr){
            sta.push(ptr) ;
            ptr=ptr->left ;
        }
    }
};

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

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