Binary Search Tree Iterator

Implement an iterator over a binary search tree (BST). Your iterator will be initialized with the root node of a BST.

Calling next() will return the next smallest number in the BST.

Note: next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.

对的

代码：

private Stack<TreeNode> stack;    private TreeNode root;    public BSTIterator(TreeNode root) {        this.root = root;        stack = new Stack<>();        pushToStack(root);    }        private void pushToStack(TreeNode node){        if(node == null) return;        TreeNode p = node;        while(p != null){            stack.push(p);            p = p.left;        }    }    /** @return whether we have a next smallest number */    public boolean hasNext() {        if(stack.isEmpty()) return false;        return true;    }    /** @return the next smallest number */    public int next() {        if(stack.isEmpty()) return -1;        TreeNode tree = stack.pop();        if(tree.right != null){            pushToStack(tree.right);        }        return tree.val;    }