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Leetcode #173: Binary Search Tree Iterator

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class BSTIterator {
    private var ar = [Int]()
    private var index = 0

    init(_ root: TreeNode?) {
        ar = inorderTraversal(root)
    }

    /** @return the next smallest number */
    func next() -> Int {
        index += 1
        return ar[index - 1]
    }

    /** @return whether we have a next smallest number */
    func hasNext() -> Bool {
        return index < ar.count
    }

    private func inorderTraversal(_ node: TreeNode?) -> [Int] {
        if node == nil {
            return []
        }
        return inorderTraversal(node?.left) + [node!.val] + inorderTraversal(node?.right)
    }
}
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