Grokking the Coding Interview: Patterns for Coding Questions
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Binary Search Tree Iterator (medium)

Problem Statement

Implement an iterator for the in-order traversal of a binary search tree (BST). That is, given a BST, we need to implement two functions:

bool hasNext(): Returns true if at least one element is left in the in-order traversal of the BST. int next(): Return the next element in the in-order traversal of the BST.


Given the following BST:

Here is the in-order traversal of this tree: [1, 4, 10, 14, 15, 19, 20]

Here is the expected output from the algorithm based on different calls:

hasNext() -> true

next() -> 1

next() -> 4




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