BFS Breadth First Search on Graph in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add the starting node to the queue in BFS.

DSA C++

std::queue<int> q;
q.[1](start);

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

Apush

Bpop

Cempty

Dfront

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop instead of push to add elements.

Using front which only accesses the first element.

Using empty which checks if the queue is empty.

In BFS, we add the starting node to the queue using the push() method.

2fill in blank

medium

Complete the code to mark a node as visited in BFS.

DSA C++

std::vector<bool> visited(n, false);
visited[[1]] = true;

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

Aqueue

Badj

Cstart

Dnode

Attempts:

3 left

💡 Hint

Common Mistakes

Marking the queue or adjacency list as visited.

Using a variable not representing a node index.

We mark the starting node as visited by setting visited[start] = true.

3fill in blank

hard

Fix the error in the BFS loop to get the current node from the queue.

DSA C++

while (!q.empty()) {
    int current = q.[1]();
    q.pop();
    // process current
}

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

Afront

Bpop

Cpush

Dempty

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() to get the element, which returns void.

Using push() or empty() incorrectly.

To get the current node without removing it, use front(). Then pop() removes it.

4fill in blank

hard

Fill both blanks to check if a neighbor is unvisited and then mark it visited.

DSA C++

for (int neighbor : adj[current]) {
    if (!visited[[1]]) {
        visited[[2]] = true;
        q.push(neighbor);
    }
}

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

Aneighbor

Bcurrent

Cstart

Dadj

Attempts:

3 left

💡 Hint

Common Mistakes

Using current or start instead of neighbor.

Using adjacency list variable instead of node index.

We check and mark the neighbor node as visited before adding it to the queue.

5fill in blank

hard

Fill all three blanks to complete BFS that prints nodes in order.

DSA C++

std::queue<int> q;
std::vector<bool> visited(n, false);
q.[1](start);
visited[[2]] = true;

while (!q.empty()) {
    int current = q.[3]();
    q.pop();
    std::cout << current << " ";
    for (int neighbor : adj[current]) {
        if (!visited[neighbor]) {
            visited[neighbor] = true;
            q.push(neighbor);
        }
    }
}

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

Apush

Bstart

Cfront

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() to get current node.

Not marking start node visited.

Using wrong variable names.

We push the start node, mark it visited, then get current node with front() in BFS.

Practice

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

easy

A. A queue data structure

B. A stack data structure

C. A priority queue

D. A hash map

Solution

1. Step 1: Understand BFS traversal method

   BFS explores nodes level by level, visiting all neighbors before moving deeper.

2. Step 2: Identify the data structure used

   To maintain the order of visiting nodes level-wise, BFS uses a queue which follows FIFO (First In First Out).

3. Final Answer:

   A queue data structure -> Option A

4. Quick Check:

   BFS uses queue = B [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 uses a hash map for order

2. Which of the following is the correct way to declare a queue for BFS traversal of integers in C++?

easy

A. vector<int> q;

B. queue<int> q;

C. stack<int> q;

D. list<int> q;

Solution

1. Step 1: Identify the correct STL container for queue

   In C++, the standard queue container is declared as queue<type>.

2. Step 2: Match the declaration for integer queue

   queue<int> q; correctly declares a queue of integers.

3. Final Answer:

   queue<int> q; -> Option B

4. Quick Check:

   Correct queue syntax = queue<int> q; [OK]

Hint: Use queue<int> for BFS integer queue in C++ [OK]

Common Mistakes:

• Using stack instead of queue
• Using vector or list which are not queues
• Missing template angle brackets

3. Consider the following graph represented as adjacency list:

0: 1, 2
1: 0, 3
2: 0, 3
3: 1, 2

If BFS starts from node 0, what is the order of nodes visited?

medium

A. [0, 1, 3, 2]

B. [0, 2, 1, 3]

C. [0, 3, 1, 2]

D. [0, 1, 2, 3]

Solution

1. Step 1: Start BFS from node 0

   Visit node 0 first, enqueue neighbors 1 and 2.

2. Step 2: Visit neighbors in queue order

   Dequeue 1, enqueue its neighbor 3 (0 already visited). Then dequeue 2, enqueue 3 but already visited.

3. Step 3: Visit node 3

   Finally dequeue 3.

4. Final Answer:

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

5. Quick Check:

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

Hint: Visit neighbors in queue order, avoid revisits [OK]

Common Mistakes:

• Visiting neighbors in wrong order
• Revisiting nodes already visited
• Confusing BFS with DFS order

4. The following BFS code snippet has a bug. What is the error?

void bfs(int start, vector& adj) {
  vector<bool> visited(adj.size(), false);
  queue<int> q;
  q.push(start);
  while (!q.empty()) {
    int node = q.front();
    q.pop();
    cout << node << " ";
    for (int neighbor : adj[node]) {
      if (!visited[neighbor]) {
        q.push(neighbor);
      }
    }
  }
}

medium

A. Neighbors are not iterated correctly

B. Queue is not initialized properly

C. Visited array is never updated to true for visited nodes

D. Start node is not pushed into the queue

Solution

1. Step 1: Check visited array usage

   Visited array is created but nodes are never marked visited after visiting.

2. Step 2: Understand impact on BFS

   Without marking visited true, nodes can be enqueued multiple times causing infinite loops or repeated visits.

3. Final Answer:

   Visited array is never updated to true for visited nodes -> Option C

4. Quick Check:

   Must mark visited[node] = true after popping [OK]

Hint: Always mark nodes visited after dequeuing [OK]

Common Mistakes:

• Forgetting to mark visited nodes
• Pushing start node multiple times
• Incorrect loop conditions

5. Given a graph with nodes 0 to 4 and edges: 0-1, 0-2, 1-3, 2-3, 3-4. Using BFS starting from node 0, which node is visited at the 3rd position in the BFS order?

hard

A. 2

B. 4

C. 1

D. 3

Solution

1. Step 1: List neighbors and BFS order

   Start at 0, enqueue neighbors 1 and 2. BFS order so far: [0]

2. Step 2: Visit nodes in queue order

   Dequeue 1 (2nd visited), enqueue 3. Dequeue 2 (3rd visited), enqueue 3 but already enqueued.

3. Step 3: Confirm 3rd visited node

   The third node visited is 2.

4. Final Answer:

   2 -> Option A

5. Quick Check:

   Third visited node = 2 [OK]

Hint: Track queue order carefully to find nth visited node [OK]

Common Mistakes:

• Mixing order of neighbors
• Counting start node incorrectly
• Ignoring repeated nodes in queue