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.

利用二叉树中序非递归遍历的思想，使用一个栈即可，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 {
    
    Stack<TreeNode>stack = new Stack<TreeNode>();

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

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return !stack.empty();
    }

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

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