Problem: 173. 二叉搜索树迭代器

思路

👩‍🏫 三叶

复杂度

时间复杂度: $O(1)$

空间复杂度: $O(h)$

Code

class BSTIterator {
Stack<TreeNode> d = new Stack<>();

	public BSTIterator(TreeNode root)
	{
		dfsLeft(root);
	}

	public int next()
	{
		TreeNode root = d.pop();
		int ans = root.val;
		root = root.right;
		dfsLeft(root);
		return ans;
	}

	public boolean hasNext()
	{
		return !d.isEmpty();
	}

	void dfsLeft(TreeNode root)
	{
		while (root != null)
		{
			d.push(root);
			root = root.left;
		}
	}
}