# Leetcode 173 Binary Search Tree Iterator.

Leetcode 173 Binary Search Tree Iterator

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.

This is a very common question towards LDR of BST.

``````
/*
* Definition for binary tree
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/

public class BSTIterator {
Stack stack;
public BSTIterator(TreeNode root) {
stack=new Stack();
while(root!=null){
stack.push(root);
root=root.left;
}
}

/* @return whether we have a next smallest number */
public boolean hasNext() {
if(!stack.isEmpty()){
return true;
}else{
return false;
}
}

/* @return the next smallest number */
public int next() {
TreeNode node=stack.pop();
int ret=node.val;
if(node.right!=null){
node=node.right;
while(node!=null){
stack.push(node);
node=node.left;
}
}
return ret;
}
}

/*
* Your BSTIterator will be called like this:
* BSTIterator i = new BSTIterator(root);
* while (i.hasNext()) v[f()] = i.next();
*/

``````