BST Inorder Successor in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return the left child of a node.

DSA Typescript

function getLeftChild(node: TreeNode | null): TreeNode | null {
  return node[1];
}

Drag options to blanks, or click blank then click option'

A.left

B.right

C.parent

D.value

Attempts:

3 left

💡 Hint

Common Mistakes

Using .right instead of .left

Using .parent or .value which are not children

The left child of a node in a BST is accessed using the .left property.

2fill in blank

medium

Complete the code to find the minimum node in a BST subtree.

DSA Typescript

function findMin(node: TreeNode | null): TreeNode | null {
  while (node && node[1]) {
    node = node.left;
  }
  return node;
}

Drag options to blanks, or click blank then click option'

A.value

B.right

C.parent

D.left

Attempts:

3 left

💡 Hint

Common Mistakes

Checking node.right instead of node.left

Returning node without checking children

To find the minimum node, keep going left until no left child exists.

3fill in blank

hard

Fix the error in the code to correctly find the inorder successor when the node has no right child.

DSA Typescript

function inorderSuccessor(root: TreeNode | null, p: TreeNode): TreeNode | null {
  let successor: TreeNode | null = null;
  while (root) {
    if (p.value < root.value) {
      successor = root;
      root = root[1];
    } else {
      root = root.right;
    }
  }
  return successor;
}

Drag options to blanks, or click blank then click option'

A.right

B.left

C.parent

D.value

Attempts:

3 left

💡 Hint

Common Mistakes

Moving right instead of left when p.value < root.value

Using root.parent which is not available

When p.value is less than root.value, move to root.left to find a smaller candidate successor.

4fill in blank

hard

Fill both blanks to complete the function that finds the inorder successor of a node in a BST.

DSA Typescript

function inorderSuccessor(root: TreeNode | null, p: TreeNode): TreeNode | null {
  if (p.right) {
    return findMin(p[1]);
  }
  let successor: TreeNode | null = null;
  while (root) {
    if (p.value < root.value) {
      successor = root;
      root = root[2];
    } else {
      root = root.right;
    }
  }
  return successor;
}

Drag options to blanks, or click blank then click option'

A.left

B.right

C.parent

D.value

Attempts:

3 left

💡 Hint

Common Mistakes

Using p.left instead of p.right for right subtree

Moving root.right instead of root.left in the loop

If p has a right child, successor is the minimum in right subtree (p.right). Otherwise, move left in root to find successor.

5fill in blank

hard

Fill all three blanks to complete the helper function and the inorder successor logic.

DSA Typescript

function findMin(node: TreeNode | null): TreeNode | null {
  while (node && node[1]) {
    node = node[2];
  }
  return node;
}

function inorderSuccessor(root: TreeNode | null, p: TreeNode): TreeNode | null {
  if (p.right) {
    return findMin(p[3]);
  }
  let successor: TreeNode | null = null;
  while (root) {
    if (p.value < root.value) {
      successor = root;
      root = root.left;
    } else {
      root = root.right;
    }
  }
  return successor;
}

Drag options to blanks, or click blank then click option'

A.left

B.right

C.parent

D.value

Attempts:

3 left

💡 Hint

Common Mistakes

Using node.right instead of node.left in findMin

Using p.left instead of p.right for successor

To find minimum, keep moving left while node.left exists. For successor, if p has right child, find minimum in p.right subtree.

Practice

1. What is the inorder successor of a node in a Binary Search Tree (BST)?

easy

A. The node with the smallest key greater than the given node's key

B. The node with the largest key smaller than the given node's key

C. The root node of the BST

D. The left child of the given node

Solution

1. Step 1: Understand inorder traversal order

   Inorder traversal visits nodes in ascending order of their keys in a BST.

2. Step 2: Define inorder successor

   The inorder successor of a node is the next node visited after it in inorder traversal, which is the smallest node with a key greater than the current node.

3. Final Answer:

   The node with the smallest key greater than the given node's key -> Option A

4. Quick Check:

   Inorder successor = next bigger node [OK]

Hint: Inorder successor is next bigger node in sorted order [OK]

Common Mistakes:

• Confusing successor with predecessor
• Thinking successor is always right child
• Assuming successor is parent node

2. Which of the following TypeScript function signatures correctly represents a function to find the inorder successor in a BST node?

easy

A. function inorderSuccessor(root: TreeNode | null, p: TreeNode): TreeNode | null

B. function inorderSuccessor(p: TreeNode): TreeNode

C. function inorderSuccessor(root: TreeNode): void

D. function inorderSuccessor(root: TreeNode, p: TreeNode): number

Solution

1. Step 1: Analyze parameters needed

   To find inorder successor, we need the BST root and the target node p.

2. Step 2: Analyze return type

   The function returns the successor node or null if none exists.

3. Final Answer:

   function inorderSuccessor(root: TreeNode | null, p: TreeNode): TreeNode | null -> Option A

4. Quick Check:

   Correct params and return type [OK]

Hint: Successor function needs root and target node, returns node or null [OK]

Common Mistakes:

• Missing root parameter
• Wrong return type (void or number)
• Not allowing null root

3. Given the BST below and the node with value 5, what is the inorder successor?

      6
     / \
    2   8
   / \   \
  0   4   9
     / \
    3   5

medium

A. Node with value 8

B. Node with value 6

C. Node with value 4

D. Node with value 9

Solution

1. Step 1: Locate node 5 and check right subtree

   Node 5 has no right child, so successor is one of its ancestors.

2. Step 2: Find lowest ancestor whose left child is ancestor of node 5

   Node 6 is the first ancestor where node 5 is in left subtree, so successor is node 6.

3. Final Answer:

   Node with value 6 -> Option B

4. Quick Check:

   Successor of 5 is 6 [OK]

Hint: If no right child, go up to first bigger ancestor [OK]

Common Mistakes:

• Choosing right child of root instead of ancestor
• Picking node 8 which is too big
• Confusing predecessor with successor

4. Consider this TypeScript code snippet to find the inorder successor of a node in a BST:

function inorderSuccessor(root: TreeNode | null, p: TreeNode): TreeNode | null {
  let successor: TreeNode | null = null;
  while (root) {
    if (p.val < root.val) {
      successor = root;
      root = root.left;
    } else {
      if (p.val === root.val) {
        return successor;
      }
      root = root.right;
    }
  }
  return successor;
}

What is the bug in this code?

medium

A. It may cause infinite loop if root is null

B. It incorrectly updates the successor when p.val > root.val

C. It does not handle the case when node p has a right subtree

D. It returns the predecessor instead of successor

Solution

1. Step 1: Understand code logic

   The code moves down the tree to find the smallest node greater than p.val, updating successor accordingly.

2. Step 2: Identify missing case

   If p has a right subtree, the successor is the leftmost node in that subtree, but this code ignores that and only searches from root.

3. Final Answer:

   It does not handle the case when node p has a right subtree -> Option C

4. Quick Check:

   Right subtree check missing [OK]

Hint: Check right subtree first for successor [OK]

Common Mistakes:

• Ignoring right subtree of p
• Assuming successor always in ancestors
• Confusing successor with predecessor

5. You have a BST and want to find the inorder successor of a node p. The node p has a right child. Which of the following is the correct approach to find the inorder successor?

hard

A. Return the root of the BST

B. Return the rightmost node in p's left subtree

C. Return the parent of p

D. Return the leftmost node in p's right subtree

Solution

1. Step 1: Check if p has right subtree

   If p has a right child, the inorder successor is the smallest node in that right subtree.

2. Step 2: Find leftmost node in right subtree

   Traverse left children starting from p's right child until no more left child exists.

3. Final Answer:

   Return the leftmost node in p's right subtree -> Option D

4. Quick Check:

   Successor in right subtree's leftmost node [OK]

Hint: Successor with right child = leftmost node in right subtree [OK]

Common Mistakes:

• Returning parent instead of leftmost right subtree node
• Returning rightmost left subtree node
• Returning root regardless of p