K’th Boom Number

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.

Input : k = 2
Output: 3

Input : k = 3
Output: 22

Input : k = 100
Output: 322323

Input: k = 1000000
Output: 3332322223223222223

Reference: http://www.spoj.com/problems/TSHOW1/
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’th Boom 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. Below is C++ implementation of this idea.





// C++ program to find K'th Boom number
using namespace std;
typedef long long int ll;
// This function uses queue data structure to K'th
// Boom number
void  boomNumber(ll k)
    // Create an empty queue of strings
    queue<string> q;
    // Enqueue an empty string
    // 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
        // Store current Boom number before changing it
        string s2 = s1;
        // Append "2" to string s1 and enqueue it
        // check if count==k
        if (count==k)
            cout << s1 << endl; // K'th Boom number
        // Append "3" to string s2 and enqueue it.
        // Note that s2 contains the previous front
        // check if count==k
        if (count==k)
            cout << s2 << endl; // K'th Boom number
    return ;
// Driver program to test above function
int main()
    ll k = 1000000;
    return 0;




This article is contributed by Shashank Mishra (Gullu). This article is reviewed by team GeeksforGeeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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