BST Iterator

BSTIterator.java

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

public class BSTIterator {
    
    Stack<TreeNode> s;
    public BSTIterator(TreeNode root) {
        s = new Stack<>();
        pushAllLeft(root);
    }

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

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

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