173. 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.

题目含义：给定一个二叉搜索树，实现next() 方法来返回下一个最小值， hasNext() 返回是否有下一个最小值

public class BSTIterator {

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

    /** @return whether we have a next smallest number */
    public boolean hasNext() {

        return !stack.isEmpty();
    }

    /** @return the next smallest number */
    public int next() {
        TreeNode tmpNode = stack.pop();
        pushAll(tmpNode.right);
        return tmpNode.val;     
    }
    
    private void pushAll(TreeNode node) {
        for (; node != null; stack.push(node), node = node.left);
    }
}