[leetcode]173. Binary Search Tree Iterator 二叉搜索树迭代器

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.

思路：

代码：

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

 public class BSTIterator {

     private TreeNode cur;
     private Stack<TreeNode> stack;

     public BSTIterator(TreeNode root) {
         cur = root;
         stack = new Stack<>();

     }

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

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

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