[Leetcode] Binary Search Tree Iterator

The solution is also available here:https://gist.github.com/pyemma/bbe39014f436f2a5aa5b

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

public class BSTIterator {
    private Stack<TreeNode> stack;
    public BSTIterator(TreeNode root) {
        stack = new Stack<TreeNode>();
        if(root != null)
            pushleft(root);
    }
    private void pushleft(TreeNode root) {
        while(root != null) {
            stack.push(root);
            root = root.left;
        }
    }
    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return !stack.isEmpty();
    }

    /** @return the next smallest number */
    public int next() {
        TreeNode temp = stack.pop();
        if(temp.right != null)
            pushleft(temp.right);
        return temp.val;
    }
}

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