Wednesday, July 11, 2012

Breadth First Search in C++ – Algorithm and Source Code

Basic Theory
Breadth – first searches are performed by exploring all nodes at a given depth before proceeding to the next level. This means that all immediate children of nodes are explored before any of the children’s children are considered. It has obvious advantage of always finding a minimal path length solution when one exists. However, a great many nodes may need to be explored before a solution is found, especially if the tree is very full.
BFS uses a queue structure to hold all generate but still unexplored nodes. The order in which nodes are placed on the queue for removal and exploration determines the type of search. The BFS algorithm proceeds as follows.

  1. Place the starting node s on the queue.
  2. If the queue is empty, return failure and stop.
  3. If the first element on the queue is a goal node g, return success and stop Otherwise,
  4. Remove and expand the first element from the queue and place all the children at the end of the queue in any order.
  5. Return to step 2.
Consider a graph
Applying above algorithm, the BFS of the graph starting from node 1 is : 1, 2, 3, 4, 6, 7, 5
Source Code

Written by Bibek Subedi


#include <iostream>
#include <ctime>
using namespace std;


 Performs the Breadth-First Graph search for both directed and
 undirected graphs. This algorithm explores all the findable nodes
 in "layers".
 @author Bibek Subedi


Class Queue represent a Queue data structure which is First In
First Out [FIFO] structured. It has operations like Enqueue which
adds an element at the rear side and Dequeue which removes the
element from front.


struct node {
    int info;
    node *next;

class Queue {
        node *front;
        node *rear;
        bool isEmpty();
        void enqueue(int);
        int dequeue();
        void display();


void Queue::display(){
    node *p = new node;
    p = front;
    if(front == NULL){
        cout<<"\nNothing to Display\n";
            p = p->next;

Queue::Queue() {
    front = NULL;
    rear = NULL;

Queue::~Queue() {
    delete front;

void Queue::enqueue(int data) {
    node *temp = new node();
    temp->info = data;
    temp->next = NULL;
    if(front == NULL){
        front = temp;
        rear->next = temp;
    rear = temp;

int Queue::dequeue() {
    node *temp = new node();
    int value;
    if(front == NULL){
        cout<<"\nQueue is Emtpty\n";
        temp = front;
        value = temp->info;
        front = front->next;
        delete temp;
    return value;

bool Queue::isEmpty() {
    return (front == NULL);


Class Graph represents a Graph [V,E] having vertices V and
edges E.

class Graph {
        int n; /// n is the number of vertices in the graph
        int **A; /// A stores the edges between two vertices
        Graph(int size = 2);
        bool isConnected(int, int);
        void addEdge(int u, int v);
        void BFS(int );

Graph::Graph(int size) {
    int i, j;
    if (size < 2) n = 2;
    else n = size;
    A = new int*[n];
    for (i = 0; i < n; ++i)
        A[i] = new int[n];
    for (i = 0; i < n; ++i)
        for (j = 0; j < n; ++j)
            A[i][j] = 0;

Graph::~Graph() {
    for (int i = 0; i < n; ++i)
    delete [] A[i];
    delete [] A;

Checks if two given vertices are connected by an edge
@param u vertex
@param v vertex
@return true if connected false if not connected
bool Graph::isConnected(int u, int v) {
    return (A[u-1][v-1] == 1);

adds an edge E to the graph G.
@param u vertex
@param v vertex
void Graph::addEdge(int u, int v) {
    A[u-1][v-1] = A[v-1][u-1] = 1;

performs Breadth First Search
@param s initial vertex
void Graph::BFS(int s) {
    Queue Q;

    /** Keeps track of explored vertices */
    bool *explored = new bool[n+1];

    /** Initailized all vertices as unexplored */
    for (int i = 1; i <= n; ++i)
    explored[i] = false;

    /** Push initial vertex to the queue */
    explored[s] = true; /** mark it as explored */
    cout << "Breadth first Search starting from vertex ";
    cout << s << " : " << endl;

    /** Unless the queue is empty */
    while (!Q.isEmpty()) {
        /** Pop the vertex from the queue */
        int v = Q.dequeue();

        /** display the explored vertices */
        cout << v << " ";

        /** From the explored vertex v try to explore all the
        connected vertices */
        for (int w = 1; w <= n; ++w)

            /** Explores the vertex w if it is connected to v
            and and if it is unexplored */
            if (isConnected(v, w) && !explored[w]) {
                /** adds the vertex w to the queue */
                /** marks the vertex w as visited */
                explored[w] = true;
    cout << endl;
    delete [] explored;

int main() {

    /** Creates a graph with 12 vertices */
    Graph g(12);

    /** Adds edges to the graph */
    g.addEdge(1, 2); g.addEdge(1, 3);
    g.addEdge(2, 4); g.addEdge(3, 4);
    g.addEdge(3, 6); g.addEdge(4 ,7);
    g.addEdge(5, 6); g.addEdge(5, 7);
    clock_t t1;
    t1 = clock();

    /** Explores all vertices findable from vertex 1 */
    float diff = (double)(clock() - t1)/CLOCKS_PER_SEC ;
    cout <<endl<< "The time taken for Breadth first search: "<< diff << endl;


The time complexity of the breadth-first search is O(bd). This can be seen by noting that all nodes up to the goal depth d are generated. Therefore, the number generated is b + b2 + . . . + bd which is O(bd). The space complexity is also O(bd) since all nodes at a given depth must be stored in order to generate the nodes at the next depth, that is, bd-1 nodes must be stored at depth d – 1 to generate nodes at depth d, which gives space complexity of O(bd). 


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  2. In your BFS function, I think you have to remove the 'continue' in the if-statement. Otherwise it's not doing the right thing. Just use for example g.BFS(1, 3); and you will see that the output after the 3rd value is different.

  3. In your bfs program remove that time thing from the void main part..

  4. main function should return an integer value ... :)

    1. Of course, but my compiler does it for me :) ... any way thank you for pointing.

    2. according to the c++ standard, it is not necessary have a return statement in main()

  5. what does this algorithm do exactly?

  6. Got a small memory leak in dequeue. You are leaking temp when you assign front to it.

  7. I think there are memory leak in the queue destructor

  8. Why did you implement your own queue anyway, when there's std::queue?

  9. I sense a memory leak .. no need to allocate memory for the temp variables.

  10. When does the display() function ever get called?

  11. post Output of the program i.e screenshot of ouput

  12. line 156 is added because of the graph is undirected, sure?

  13. Thank a lot.
    The style you code is neat and perspicuous.
    It helps me.