leetcode------Binary Search Tree Iterator

标题：	Binary Search Tree Iterator
通过率：	28.9%
难度：	中等

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.

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

就是非递归中序遍历二叉树：

代码如下：

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

public class BSTIterator {
    TreeNode node=null;
    Stack<TreeNode> stack=new Stack<TreeNode>();
    public BSTIterator(TreeNode root) {
        if(root!=null){
            stack.push(root);
            node=root;
            while(node.left!=null){
                stack.push(node.left);
                node=node.left;
            }
        }
    }

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        if(!stack.isEmpty())return true;
        else return false;
    }

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

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