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.

把中序遍历拆分成hasnext()和next()。

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class BSTIterator {    TreeNode node;    TreeNode root;    Stack<TreeNode> stack;    public BSTIterator(TreeNode root) {        this.root=root;        node=root;        stack=new Stack<>();    }    /** @return whether we have a next smallest number */    public boolean hasNext() {        return  node!=null||!stack.isEmpty();    }    /** @return the next smallest number */    public int next() {        TreeNode res=null;        while(node!=null){            stack.push(node);            node=node.left;        }        if(!stack.isEmpty()){            node=stack.pop();            res=node;            node=node.right;        }        return res.val;    }}/** * Your BSTIterator will be called like this: * BSTIterator i = new BSTIterator(root); * while (i.hasNext()) v[f()] = i.next(); */