173 BST 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.

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

public class BSTIterator {
    Stack<TreeNode> stack = null;
    TreeNode currentNode = null;

    public BSTIterator(TreeNode root) {
        stack = new Stack<>();
        currentNode = root;
        while(currentNode != null) {
            stack.push(currentNode);
            currentNode = currentNode.left;
        }
    }

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        if (stack.isEmpty()) return false;
        currentNode = stack.pop();
        if (currentNode.right != null) {
            TreeNode node = currentNode.right;
            while(node != null) {
                stack.push(node);
                node = node.left;
            }
        }
        return true;
    }

    /** @return the next smallest number */
    public int next() {
        return currentNode.val;
    }
}

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