Binary Search Tree Iterator

Binary Search Tree Iterator stack

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.

Binary Search Tree Iterator Solution

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

public class BinarySearchTreeIterator {
    Stack<TreeNode> stack;

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

        while(root != null) {
            stack.push(root);
            root = root.left;
        }
    }

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return stack.isEmpty() ? false : true;
    }

    /** @return the next smallest number */
    public int next() {
        TreeNode nextSmallest = stack.pop();
        TreeNode addToStack = nextSmallest.right;

        while(addToStack != null) {
            stack.add(addToStack);
            addToStack = addToStack.left;
        }

        return nextSmallest.val;
    }
}

Last modified October 4, 2020