BSTIterator.java

import java.util.Stack;

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

    Stack<TreeNode> stack;

    public BSTIterator(TreeNode root) {
        stack = new Stack<TreeNode>();
        pushAll(root);
    }

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

    /** @return the next smallest number */
    public int next() {
        TreeNode smlestNode = stack.pop();
        pushAll(smlestNode.right);
        return smlestNode.val;
    }

    public void pushAll(TreeNode node) {
        for(; node != null; stack.push(node), node = node.left);
    }
}

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