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;

    }

}

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