Binary Search Tree Iterator 二叉搜索树的迭代器

题目：

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.

用java实现：

// Definition for binary tree
class TreeNode {
int val;
TreeNode left;
TreeNode right;
TreeNode(int x) { val = x; }
}

public class BSTIterator {

private Stack<TreeNode> stack = new Stack<TreeNode>();
private TreeNode current = null;
public BSTIterator(TreeNode root) {
current = root;
}

/** @return whether we have a next smallest number */
public boolean hasNext() {
return (current != null || !stack.isEmpty());
}

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

/**
* Your BSTIterator will be called like this:
* BSTIterator i = new BSTIterator(root);
* while (i.hasNext()) v[f()] = i.next();
*/