173. 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.

This is another version of in order traversal of tree.

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

public class BSTIterator {
    TreeNode node;
    Stack<TreeNode> stack = new Stack<>();

    public BSTIterator(TreeNode root) {
        node = root;
    }

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

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

      return top.val;

    }
}

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