BFS Breadth First Search on Graph in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the queue with the start node.

DSA Typescript

const queue: number[] = [[1]];

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

Agraph

B0

CstartNode

Dvisited

Attempts:

3 left

💡 Hint

Common Mistakes

Adding the whole graph or visited set instead of the start node.

Starting with an empty queue.

We start BFS by adding the start node to the queue to begin traversal.

2fill in blank

medium

Complete the code to mark the current node as visited.

DSA Typescript

visited[[1]] = true;

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Agraph

Bqueue

CstartNode

Dnode

Attempts:

3 left

💡 Hint

Common Mistakes

Marking the queue or graph as visited instead of the node.

Not marking nodes as visited, causing infinite loops.

We mark the current node as visited to avoid revisiting it.

3fill in blank

hard

Fix the error in the loop that processes neighbors of the current node.

DSA Typescript

for (const neighbor of graph[[1]]) {

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Anode

Bqueue

Cvisited

DstartNode

Attempts:

3 left

💡 Hint

Common Mistakes

Using queue or visited as index causes errors or wrong neighbors.

Using startNode instead of current node inside the loop.

We iterate over neighbors of the current node, so we use 'node' as the index.

4fill in blank

hard

Fill both blanks to check if a neighbor is unvisited and add it to the queue.

DSA Typescript

if (!visited[[1]]) {
  queue.[2](neighbor);
}

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Aneighbor

Bpush

Cpop

Dnode

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop instead of push adds/removes wrong elements.

Checking visited status of wrong variable.

We check if the neighbor is not visited and then add it to the queue using push.

5fill in blank

hard

Fill all three blanks to complete the BFS function that returns the order of visited nodes.

DSA Typescript

function bfs(graph: number[][], startNode: number): number[] {
  const visited: boolean[] = new Array(graph.length).fill(false);
  const queue: number[] = [startNode];
  const order: number[] = [];

  while (queue.length > 0) {
    const [1] = queue.shift()!;
    if (!visited[[2]]) {
      visited[[3]] = true;
      order.push(node);

      for (const neighbor of graph[node]) {
        if (!visited[neighbor]) {
          queue.push(neighbor);
        }
      }
    }
  }
  return order;
}

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

Anode

BstartNode

Dqueue

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently causes errors.

Not marking nodes as visited before processing neighbors.

We assign the shifted node to 'node', check and mark visited[node], and use 'node' consistently.

Practice

1. What does BFS (Breadth First Search) primarily use to keep track of nodes to visit next in a graph?

easy

A. A priority queue to select nodes by weight

B. A stack to process nodes in depth order

C. A hash map to store visited nodes only

D. A queue to process nodes level by level

Solution

1. Step 1: Understand BFS traversal method

   BFS explores nodes level by level, so it needs a structure that processes nodes in the order they are discovered.

2. Step 2: Identify the data structure used

   A queue follows First-In-First-Out (FIFO) order, which matches BFS's level order traversal.

3. Final Answer:

   A queue to process nodes level by level -> Option D

4. Quick Check:

   BFS uses a queue = A [OK]

Hint: BFS uses FIFO order, so it needs a queue [OK]

Common Mistakes:

• Confusing BFS with DFS which uses a stack
• Thinking BFS uses a priority queue
• Assuming BFS only uses a visited set without a queue

2. Which of the following is the correct TypeScript syntax to initialize a queue for BFS using an array?

easy

A. const queue = new Queue<number>();

B. const queue: number[] = [];

C. let queue = {};

D. var queue = new ArrayQueue();

Solution

1. Step 1: Recognize TypeScript array initialization

   To create a queue using an array, declare an array with type annotation and initialize it empty.

2. Step 2: Check each option

   const queue: number[] = []; correctly declares an empty number array. const queue = new Queue<number>(); uses a class Queue which is not built-in. let queue = {}; initializes an object, not a queue. var queue = new ArrayQueue(); uses a non-existent ArrayQueue.

3. Final Answer:

   const queue: number[] = []; -> Option B

4. Quick Check:

   Array queue syntax = A [OK]

Hint: Use typed empty array for queue in TypeScript [OK]

Common Mistakes:

• Using undefined classes like Queue without import
• Initializing queue as an object {} instead of array
• Using var instead of const or let (less preferred)

3. Given the graph adjacency list below and BFS starting from node 0, what is the order of nodes visited?

const graph = {
  0: [1, 2],
  1: [3],
  2: [3],
  3: []
};

medium

A. [0, 1, 2, 3]

B. [0, 2, 1, 3]

C. [0, 3, 1, 2]

D. [1, 2, 0, 3]

Solution

1. Step 1: Start BFS from node 0

   Visit node 0 first, then enqueue its neighbors 1 and 2.

2. Step 2: Visit neighbors in queue order

   Dequeue 1, visit it, enqueue its neighbor 3. Then dequeue 2, visit it, enqueue 3 again but already visited. Finally, dequeue 3.

3. Final Answer:

   [0, 1, 2, 3] -> Option A

4. Quick Check:

   BFS order = [0, 1, 2, 3] [OK]

Hint: Visit nodes level by level, enqueue neighbors in order [OK]

Common Mistakes:

• Visiting neighbors in wrong order
• Visiting node 3 twice
• Starting BFS from wrong node

4. Consider this BFS code snippet in TypeScript. What is the error?

function bfs(graph: Record, start: number) {
  const queue: number[] = [start];
  const visited = new Set<number>();
  while (queue.length > 0) {
    const node = queue.shift();
    if (!visited.has(node)) {
      visited.add(node);
      for (const neighbor of graph[node]) {
        queue.push(neighbor);
      }
    }
  }
  return Array.from(visited);
}

medium

A. queue.shift() can return undefined, causing errors

B. visited set is not updated correctly

C. Neighbors are not added to queue

D. Function does not return anything

Solution

1. Step 1: Analyze queue.shift() usage

   queue.shift() returns number | undefined. If queue is empty, node can be undefined, causing errors when accessing graph[node].

2. Step 2: Identify missing check for undefined

   Code should check if node is undefined before using it. Without this, runtime error can occur.

3. Final Answer:

   queue.shift() can return undefined, causing errors -> Option A

4. Quick Check:

   Check for undefined after shift = B [OK]

Hint: Always check queue.shift() result for undefined [OK]

Common Mistakes:

• Ignoring possible undefined from shift()
• Assuming visited set is wrong when it is correct
• Thinking neighbors are not added when they are

5. You want to find the shortest path length from node 0 to node 4 in this graph using BFS:

const graph = {
  0: [1, 2],
  1: [3],
  2: [3, 4],
  3: [4],
  4: []
};

Which BFS modification helps find the shortest path length correctly?

hard

A. Ignore visited set to explore all paths

B. Use a stack instead of a queue

C. Track distance for each node when first visited

D. Visit nodes multiple times to update distance

Solution

1. Step 1: Understand BFS for shortest path

   BFS finds shortest paths in unweighted graphs by exploring neighbors level by level.

2. Step 2: Track distance when visiting nodes

   Maintain a distance map or array to record the number of edges from start node to each node when first visited.

3. Final Answer:

   Track distance for each node when first visited -> Option C

4. Quick Check:

   Distance tracking in BFS = D [OK]

Hint: Store distance on first visit to each node [OK]

Common Mistakes:

• Using stack breaks BFS order
• Visiting nodes multiple times wastes time
• Ignoring visited set causes infinite loops