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.

Callingnext()will return the next smallest number in the BST.

Note:next()andhasNext()should run in average O(1) time and uses O(h) memory, wherehis the height of the tree.

题意： 用栈来实现二叉树的中序遍历

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();
 */