/** * 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(); */