leetcode 173. Binary Search Tree Iterator

题目描述：

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.

 

Example:

BSTIterator iterator = new BSTIterator(root);
iterator.next();    // return 3
iterator.next();    // return 7
iterator.hasNext(); // return true
iterator.next();    // return 9
iterator.hasNext(); // return true
iterator.next();    // return 15
iterator.hasNext(); // return true
iterator.next();    // return 20
iterator.hasNext(); // return false

思路：

二叉树搜索树的中序遍历是升序序列。

实现：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.Stack;
class BSTIterator {
    TreeNode curr=null;
    Stack<TreeNode> s=new Stack<>();
    public BSTIterator(TreeNode root) {
        curr=root;
    }
    
    /** @return the next smallest number */
    public int next() {
        while(curr!=null){
            s.push(curr);
            curr=curr.left;
        }
        curr=s.pop();
        TreeNode node=curr;
        curr=curr.right;
        return node.val;
    }
    
    /** @return whether we have a next smallest number */
    public boolean hasNext() {
         return (curr!=null||!s.isEmpty());
    }
}