LeetCode 230 - Kth Smallest Element in a BST with Code

The “Kth Smallest Element in a BST” problem on LeetCode is a popular problem for practicing and improving your coding skills. It involves finding the kth smallest element in a binary search tree (BST).

Here is a step-by-step guide on how to solve the “Kth Smallest Element in a BST” problem on LeetCode, including explanations for each line of code.

Understand the problem:

The first step to solving any problem is to thoroughly understand the problem statement. In this case, the problem involves finding the kth smallest element in a BST. Make sure you understand the input, output, and any constraints or limitations of the problem.

Plan your solution:

Now that you understand the problem, it’s time to come up with a plan to solve it. One approach to solving this problem is to use an in-order traversal of the BST, which visits the nodes in the tree in ascending order. You can keep track of the position of the current node in the traversal and return the kth smallest element when you reach it.

Implement your solution:

Now that you have a plan, it’s time to start coding. Begin by writing a function that takes in a BST and an integer k as input and returns the kth smallest element in the BST. You can use a recursive in-order traversal to visit the nodes of the tree in ascending order and keep track of the position of the current node in the traversal. Here is an example of how to implement this solution in Python.

def kthSmallest(root, k):
    # Initialize a counter to keep track of the position of the current node
    # in the in-order traversal
    counter = [0]

    # Recursive function to perform in-order traversal of the BST
    def in_order(node):
        # Base case: if the node is None, return None
        if not node:
            return None

        # Recursively traverse the left subtree
        left = in_order(node.left)
        if left is not None:
            return left

        # Increment the counter and check if the current node is the kth smallest
        counter[0] += 1
        if counter[0] == k:
            return node.val

        # Recursively traverse the right subtree
        right = in_order(node.right)
        if right is not None:
            return right

    # Call the recursive function to perform the in-order traversal and return the result
    return in_order(root)

Test your solution:

It’s important to test your solution to make sure it is correct. LeetCode provides test cases that you can use to verify that your solution is correct. You can also write some test cases of your own to ensure that your solution is correct for all possible inputs.

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