K'th Boom Number - GeeksforGeeks

K'th Boom Number - GeeksforGeeks
Boom numbers are numbers consisting only of digits 2 and 3. Given an integer k (0<k<=10^7) , display the k-th Boom number.

The idea is very simple like Generate Binary Numbers . Here also we use same approach ,
we use queue data structure to solve this problem. First enqueue “2” then “3” these two are first and second boom number respectively. Now set count=2, for each time pop() front of queue and append “2” in poped number and increment count++ if (count==k) then print current Boom number else append “3” in poped number and increment count++ if (count==k) then print current Boom number. Repeat the process untill we reach to K’thBoom number.

This approach can be seen as BFS of a tree with root as empty string. Left child of every node has a 2 appended and right child has 3 appended.

void  boomNumber(ll k)

{

    // Create an empty queue of strings

    queue<string> q;

 

    // Enqueue an empty string

    q.push("");

 

    // counter for K'th element

    ll count = 0;

 

    // This loop checks the value of count to

    // become equal to K when value of count

    // will be equals to k we will print the

    // Boom number

    while (count <= k)

    {

        // current Boom number

        string s1 = q.front();

 

        // pop front

        q.pop();

 

        // Store current Boom number before changing it

        string s2 = s1;

 

        // Append "2" to string s1 and enqueue it

        q.push(s1.append("2"));

        count++;

 

        // check if count==k

        if (count==k)

        {

            cout << s1 << endl; // K'th Boom number

            break;

        }

 

        // Append "3" to string s2 and enqueue it.

        // Note that s2 contains the previous front

        q.push(s2.append("3"));

        count++;

 

        // check if count==k

        if (count==k)

        {

            cout << s2 << endl; // K'th Boom number

            break;

        }

    }

    return ;

}

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