Java for LeetCode 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.

解题思路：

其实就是按照中序遍历的顺序出列，有两种实现思路：

一、构造BST的时候直接预处理下，构造出一条按照中序遍历的list，每次读取list的元素即可。

二、不进行预处理，按照中序遍历的思路使用Stack动态的进行处理

这里我们实现思路二：

public class BSTIterator {
	private Stack<TreeNode> stack=new Stack<TreeNode>();

	public BSTIterator(TreeNode root) {
		if (root != null)
			pushLeft(root);
	}

	/** @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.peek();
		stack.pop();
		if(top.right!=null)
		pushLeft(top.right);
		return top.val;
	}

	public void pushLeft(TreeNode root) {
		stack.push(root);
		TreeNode rootTemp = root.left;
		while (rootTemp != null) {
			stack.push(rootTemp);
			rootTemp = rootTemp.left;
		}
	}
}