kth smallest

kth_smallest.py

def kthSmallest(root, k):
    """
    :type root: TreeNode
    :type k: int
    :rtype: Optional[int]
    """
    def recurse(node, k):
        if not node:
            return None, k

        left, k = recurse(node.left,k)
        if left is not None:
            return left, k

        k -= 1
        if k == 0:
            return node.val, k

        return recurse(node.right, k)

    value, k = recurse(root, k)
    return value