Problem: Binary Search Tree Iterator

Question

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.

For example: Given the binary search tree,

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

Note:

next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.
You may assume that next() call will always be valid, that is, there will be at least a next smallest number in the BST when next() is called.

Solution

TODO

Use a stack do inorder traversal.

Code

Here is a sample solution.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class BSTIterator {
    Stack<TreeNode> s;
    public BSTIterator(TreeNode root) {
        s = new Stack<TreeNode>();
        collectLeftChildren(root);
    }

    /** @return the next smallest number */
    public int next() {
        TreeNode n = s.pop();
        collectLeftChildren(n.right);
        return n.val;
    }

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return !s.isEmpty();
    }

    public void collectLeftChildren(TreeNode node) {
        while(node != null) {
            s.push(node);
            node = node.left;
        }
    }
}

/**
 * Your BSTIterator object will be instantiated and called as such:
 * BSTIterator obj = new BSTIterator(root);
 * int param_1 = obj.next();
 * boolean param_2 = obj.hasNext();
 */

Performance

next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.

Problem: Binary Search Tree Iterator

Question

Solution

Code

Performance

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