Binary Search Tree Iterator

Binary Search Tree Iterator

问题：

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.

思路:

　　中序遍历的非递归算法实现 栈

我的代码：

public class BSTIterator {
    Stack<TreeNode> stack = new Stack<TreeNode>();
    public BSTIterator(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 top = stack.pop();
        TreeNode right = top.right;
        if(right != null)
        {
            while(right != null)
            {
                stack.push(right);
                right = right.left;
            }
        }
        return top.val;
    }
}