Kth Smallest Element in BST in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the function that finds the kth smallest element in a BST.

DSA C++

int kthSmallest(TreeNode* root, int k) {
    int count = 0;
    int result = 0;
    inorder(root, k, count, [1]);
    return result;
}

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Acount

Broot

Ck

Dresult

Attempts:

3 left

💡 Hint

Common Mistakes

Passing count instead of result by reference.

Not passing any variable by reference.

The inorder traversal function updates the result variable when the kth element is found.

2fill in blank

medium

Complete the inorder traversal code to find the kth smallest element.

DSA C++

void inorder(TreeNode* root, int k, int& count, int& result) {
    if (!root) return;
    inorder(root->left, k, count, result);
    count++;
    if (count == [1]) {
        result = root->val;
        return;
    }
    inorder(root->right, k, count, result);
}

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Ak

Bcount

Cresult

Droot->val

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing count with result instead of k.

Using root->val in the condition.

The traversal checks if the current count matches k to find the kth smallest element.

3fill in blank

hard

Fix the error in the base case of the inorder traversal function.

DSA C++

void inorder(TreeNode* root, int k, int& count, int& result) {
    if (root == [1]) return;
    inorder(root->left, k, count, result);
    count++;
    if (count == k) {
        result = root->val;
        return;
    }
    inorder(root->right, k, count, result);
}

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Anullptr

Bfalse

CNULL

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or NULL instead of nullptr.

Using false which is a boolean, not a pointer.

In modern C++, nullptr is the correct way to check for a null pointer.

4fill in blank

hard

Fill both blanks to complete the iterative inorder traversal to find the kth smallest element.

DSA C++

int kthSmallest(TreeNode* root, int k) {
    std::stack<TreeNode*> stack;
    TreeNode* current = root;
    int count = 0;
    while (current != [1] || !stack.[2]()) {
        while (current != nullptr) {
            stack.push(current);
            current = current->left;
        }
        current = stack.top();
        stack.pop();
        count++;
        if (count == k) return current->val;
        current = current->right;
    }
    return -1;
}

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Anullptr

Bempty

Csize

Dtop

Attempts:

3 left

💡 Hint

Common Mistakes

Using stack.size() instead of stack.empty().

Using NULL instead of nullptr.

The loop continues while current is not nullptr or the stack is not empty.

5fill in blank

hard

Fill all three blanks to complete the recursive helper function signature and call.

DSA C++

void [1](TreeNode* root, int k, int& count, int& result) {
    if (root == nullptr) return;
    [1](root->left, k, count, result);
    count++;
    if (count == k) {
        result = root->val;
        return;
    }
    [1](root->right, k, count, result);
}

int kthSmallest(TreeNode* root, int k) {
    int count = 0;
    int result = 0;
    [2](root, k, count, result);
    return result;
}

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Ainorder

BinorderTraversal

CkthSmallestHelper

Dtraverse

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the helper function in definition and calls.

Using the main function name as helper.

The helper function is named consistently as inorder in both definition and calls.