## Binary Search Tree Iterator

07/16/2016 Tree Stack Design

## 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.

## Solution

Result: Accepted Time: 32 ms

Here should be some explanations.

/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class BSTIterator {
vector<TreeNode*> stck;
public:
BSTIterator(TreeNode *root) {
while(root)
{
stck.push_back(root);
root = root->left;
}
}

/** @return whether we have a next smallest number */
bool hasNext() {
return stck.size() > 0;
}

/** @return the next smallest number */
int next() {
TreeNode * tmp = stck.back();
stck.pop_back();
int ret = tmp->val;
tmp = tmp->right;
while(tmp)
{
stck.push_back(tmp);
tmp = tmp->left;
}
return ret;
}
};

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


Complexity Analytics

• Time Complexity: $O(1)$
• Space Complexity: $O(h)$