Two Sum in BST in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Two Sum in BST with target 9

What is the output of the following code that checks if there exist two nodes in the BST whose values sum to 9?

DSA Javascript

class TreeNode {
  constructor(val, left = null, right = null) {
    this.val = val;
    this.left = left;
    this.right = right;
  }
}

function findTarget(root, k) {
  const set = new Set();
  function dfs(node) {
    if (!node) return false;
    if (set.has(k - node.val)) return true;
    set.add(node.val);
    return dfs(node.left) || dfs(node.right);
  }
  return dfs(root);
}

const root = new TreeNode(5,
  new TreeNode(3,
    new TreeNode(2),
    new TreeNode(4)
  ),
  new TreeNode(6,
    null,
    new TreeNode(7)
  )
);

console.log(findTarget(root, 9));

A TypeError

B false

C undefined

D true

💡 Hint

Think about whether any two nodes add up to 9 in the BST.

The nodes with values 2 and 7 sum to 9, so the function returns true.

❓ Predict Output

intermediate

Output of Two Sum in BST with target 28

What is the output of the following code that checks if there exist two nodes in the BST whose values sum to 28?

DSA Javascript

class TreeNode {
  constructor(val, left = null, right = null) {
    this.val = val;
    this.left = left;
    this.right = right;
  }
}

function findTarget(root, k) {
  const set = new Set();
  function dfs(node) {
    if (!node) return false;
    if (set.has(k - node.val)) return true;
    set.add(node.val);
    return dfs(node.left) || dfs(node.right);
  }
  return dfs(root);
}

const root = new TreeNode(10,
  new TreeNode(5,
    new TreeNode(3),
    new TreeNode(7)
  ),
  new TreeNode(15,
    null,
    new TreeNode(18)
  )
);

console.log(findTarget(root, 28));

A true

B false

C null

D ReferenceError

💡 Hint

Check if any two nodes add up to 28 in the BST.

The nodes with values 10 and 18 sum to 28, so the function returns true.

🔧 Debug

advanced

Identify the error in Two Sum in BST implementation

What error will this code produce when run?

DSA Javascript

class TreeNode {
  constructor(val, left = null, right = null) {
    this.val = val;
    this.left = left;
    this.right = right;
  }
}

function findTarget(root, k) {
  const set = new Set();
  function dfs(node) {
    if (!node) return false;
    if (set.has(k - node.val)) return true;
    set.add(node.val);
    return dfs(node.left) || dfs(node.right);
  }
  return dfs(root);
}

const root = new TreeNode(1,
  new TreeNode(0),
  new TreeNode(3)
);

console.log(findTarget(root, "3"));

A No error, outputs true

B true

C false

D TypeError

💡 Hint

Consider JavaScript type coercion in arithmetic operations.

No error is produced. JavaScript coerces the string "3" to number 3 during subtraction, and finds nodes 0 and 3 sum to 3, outputting true.

🧠 Conceptual

advanced

Why use a Set in Two Sum in BST?

Why does the Two Sum in BST algorithm use a Set to store node values during traversal?

A To count the frequency of each node value

B To sort the node values for binary search

C To keep track of visited nodes and check complements efficiently

D To store nodes in insertion order for traversal

💡 Hint

Think about how to quickly check if a complement value exists.

The Set allows O(1) lookup to check if the complement (target - current node value) exists among visited nodes.

🚀 Application

expert

Number of pairs summing to target in BST

Given the BST below, how many unique pairs of nodes sum to 10? BST structure: 5 / \ 3 7 / \ \ 2 4 8

A 1

B 2

C 3

D 4

💡 Hint

List all pairs and count those summing to 10.

Pairs (2,8) and (3,7) sum to 10. The pair (4,6) is invalid as 6 is not in the tree. Since 5 appears once, (5,5) is not valid. So total 2 pairs.