Binary Search Tree Iterator

Binary Search Tree Iterator
- 题目描述
- 解法

Binary Search Tree Iterator

题目描述

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.

解法

代码如下:

class BSTIterator {

    Stack<TreeNode> s = new Stack<>();

    public BSTIterator(TreeNode root) {
        while( root != null ) {
            s.push( root );
            root = root.left;
        }
    }

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return !s.isEmpty();
    }

    /** @return the next smallest number */
    public int next() {
        TreeNode node = s.pop();
        TreeNode right = node.right;
        while( right != null ) {
            s.push( right );
            right = right.left;
        }
        return node.val;
    }
}

思考过程: 迭代器在这里等价于进行中序遍历, 使用栈实现中序遍历即可.