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Convert to a string that is repetition of a substring of k length

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  • Difficulty Level : Medium
  • Last Updated : 06 Jul, 2022
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Given a string, find if it is possible to convert it to a string that is the repetition of a substring with k characters. To convert, we can replace one substring of length k starting at index i (zero-based indexing) such that i is divisible by K, with k characters.

Examples:  

Input: str = "bdac",  k = 2
Output: True
We can either replace "bd" with "ac" or 
"ac" with "bd".

Input: str = "abcbedabcabc",  k = 3
Output: True
Replace "bed" with "abc" so that the 
whole string becomes repetition of "abc".

Input: str = "bcacc", k = 3
Output: False
k doesn't divide string length i.e. 5%3 != 0

Input: str = "bcacbcac", k = 2
Output: False

Input: str = "bcdbcdabcedcbcd", k = 3
Output: False

This can be used in compression. If we have a string where the complete string is repetition except one substring, then we can use this algorithm to compress the string.

One observation is, length of string must be a multiple of k as we can replace only one substring. 

The idea is to declare a map mp which maps strings of length k to an integer denoting its count. So, if there are only two different sub-strings of length k in the map container and the count of one of the sub-string is 1 then the answer is true. Otherwise, answer is false. 

Implementation:

C++




// C++ program to check if a string can be converted to
// a string that has repeated substrings of length k.
#include<bits/stdc++.h>
using namespace std;
 
// Returns true if str can be converted to a string
// with k repeated substrings after replacing k
// characters.
bool checkString(string str, long k)
{
    // Length of string must be a multiple of k
    int n = str.length();
    if (n%k != 0)
        return false;
 
    // Map to store strings of length k and their counts
    unordered_map<string, int> mp;
    for (int i=0; i<n; i+=k)
        mp[str.substr(i, k)]++;
 
    // If string is already a repetition of k substrings,
    // return true.
    if (mp.size() == 1)
        return true;
 
    // If number of distinct substrings is not 2, then
    // not possible to replace a string.
    if (mp.size() != 2)
        return false;
 
    // One of the two distinct must appear exactly once.
    // Either the first entry appears once, or it appears
    // n/k-1 times to make other substring appear once.
    if ((mp.begin()->second == (n/k - 1)) ||
                    mp.begin()->second == 1)
       return true;
 
    return false;
}
 
// Driver code
int main()
{
    checkString("abababcd", 2)? cout << "Yes" :
                                cout << "No";
    return 0;
}

Java




// Java program to check if a string
// can be converted to a string that has
// repeated substrings of length k.
import java.util.HashMap;
import java.util.Iterator;
 
class GFG
{
 
    // Returns true if str can be converted
    // to a string with k repeated substrings
    // after replacing k characters.
    static boolean checkString(String str, int k)
    {
 
        // Length of string must be
        // a multiple of k
        int n = str.length();
        if (n % k != 0)
            return false;
 
        // Map to store strings of
        // length k and their counts
        HashMap<String, Integer> mp = new HashMap<>();
        try
        {
            for (int i = 0; i < n; i += k)
                mp.put(str.substring(i, k),
                mp.get(str.substring(i, k)) == null ? 1 :
                mp.get(str.substring(i, k)) + 1);
        } catch (Exception e) {    }
 
        // If string is already a repetition
        // of k substrings, return true.
        if (mp.size() == 1)
            return true;
 
        // If number of distinct substrings is not 2,
        // then not possible to replace a string.
        if (mp.size() != 2)
            return false;
 
        HashMap.Entry<String,
                Integer> entry = mp.entrySet().iterator().next();
 
        // One of the two distinct must appear
        // exactly once. Either the first entry
        // appears once, or it appears n/k-1 times
        // to make other substring appear once.
        if (entry.getValue() == (n / k - 1) ||
            entry.getValue() == 1)
            return true;
 
        return false;
    }
 
    // Driver code
    public static void main(String[] args)
    {
        if (checkString("abababcd", 2))
            System.out.println("Yes");
        else
            System.out.println("No");
    }
}
 
// This code is contributed by
// sanjeev2552

Python3




# Python3 program to check if a can be converted to
# a that has repeated subs of length k.
 
# Returns True if S can be converted to a
# with k repeated subs after replacing k
# characters.
def check( S, k):
     
    # Length of must be a multiple of k
    n = len(S)
     
    if (n % k != 0):
        return False
 
    # Map to store s of length k and their counts
    mp =  {}
    for i in range(0, n, k):
        mp[S[i:k]] = mp.get(S[i:k], 0) + 1
 
    # If is already a repetition of k subs,
    # return True.
    if (len(mp) == 1):
        return True
 
    # If number of distinct subs is not 2, then
    # not possible to replace a .
    if (len(mp) != 2):
        return False
 
    # One of the two distinct must appear exactly once.
    # Either the first entry appears once, or it appears
    # n/k-1 times to make other sub appear once.
    for i in mp:
        if i == (n//k - 1) or mp[i] == 1:
            return True
 
    return False
 
# Driver code
 
if check("abababcd", 2):
    print("Yes")
else:
    print("No")
     
# This code is contributed by mohit kumar 29   

C#




// C# program to check if a string
// can be converted to a string that has
// repeated substrings of length k.
using System;
using System.Collections.Generic;
 
class GFG
{
 
    // Returns true if str can be converted
    // to a string with k repeated substrings
    // after replacing k characters.
    static bool checkString(String str, int k)
    {
 
        // Length of string must be
        // a multiple of k
        int n = str.Length;
        if (n % k != 0)
            return false;
 
        // Map to store strings of
        // length k and their counts
        Dictionary<String,
                   int> mp = new Dictionary<String,
                                            int>();
 
        for (int i = 0; i < n; i += k)
        {
            if(!mp.ContainsKey(str.Substring(i, k)))
                mp.Add(str.Substring(i, k), 1);
            else
                mp[str.Substring(i, k)] = mp[str.Substring(i, k)] + 1;
        }
 
        // If string is already a repetition
        // of k substrings, return true.
        if (mp.Count == 1)
            return true;
 
        // If number of distinct substrings is not 2,
        // then not possible to replace a string.
        if (mp.Count != 2)
            return false;
 
        foreach(KeyValuePair<String, int> entry in mp)
        {
 
            // One of the two distinct must appear
            // exactly once. Either the first entry
            // appears once, or it appears n/k-1 times
            // to make other substring appear once.
            if (entry.Value == (n / k - 1) ||
                entry.Value == 1)
                return true;
        }
        return false;
    }
 
    // Driver code
    public static void Main(String[] args)
    {
        if (checkString("abababcd", 2))
            Console.WriteLine("Yes");
        else
            Console.WriteLine("No");
    }
}
 
// This code is contributed by Princi Singh

Javascript




<script>
// Javascript program to check if a string
// can be converted to a string that has
// repeated substrings of length k.
     
    // Returns true if str can be converted
    // to a string with k repeated substrings
    // after replacing k characters.
    function checkString(str,k)
    {
        // Length of string must be
        // a multiple of k
        let n = str.length;
        if (n % k != 0)
            return false;
   
        // Map to store strings of
        // length k and their counts
        let mp = new Map();
        for (let i = 0; i < n; i += k)        
        {
            if(mp.has(str.substring(i, i+k)))
                mp.set(str.substring(i, i+k),mp.get(str.substring(i, i+k)) + 1);
            else
                mp.set(str.substring(i, i+k),1);
        }    
   
        // If string is already a repetition
        // of k substrings, return true.
        if (mp.size == 1)
            return true;
   
        // If number of distinct substrings is not 2,
        // then not possible to replace a string.
        if (mp.size != 2)
        {   
            return false;
          }
         
   
        // One of the two distinct must appear
        // exactly once. Either the first entry
        // appears once, or it appears n/k-1 times
        // to make other substring appear once.
        for (let [key, value] of mp.entries())
        {
             
            if(value == (Math.floor(n/k) - 1) || value == 1)
                return true;
        }
   
        return false;
    }
     
    // Driver code
    if (checkString("abababcd", 2))
        document.write("Yes");
    else
        document.write("No");
     
     
    // This code is contributed by unknown2108
</script>

Output

Yes

Time complexity : O(n) 
Auxiliary Space : O(n)

This article is contributed by Himanshu Gupta(Bagri). If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. 


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