173. Binary Search Tree Iterator

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

public class BSTIterator {
    Stack<TreeNode> stack=null;
    public BSTIterator(TreeNode root) {
        stack=new Stack<TreeNode>();
        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() {
        int res=0;
        if(!stack.isEmpty())
        {
            TreeNode node=stack.pop();
            res=node.val;
            node=node.right;
            while(node!=null)
            {
                stack.push(node);
                node=node.left;
            }
        }
        return res;
    }
}

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