Modify the string by inserting characters so that each substring of length K contains only unique characters

When processing strings, a common task is to ensure that the string meets certain conditions. One of the conditions might be to ensure that each substring of length K in the string contains only unique characters. This is a common requirement in issues related to data encoding, string manipulation, and cryptography.

Problem Statement

The problem we are trying to solve can be stated as follows -

Given a string str and an integer K, modify the string by inserting characters so that each substring of length K in the string contains only unique characters.

Suggested solutions

We can solve this problem by using sliding window technology, which is a method to efficiently check the properties of consecutive subarrays or substrings in a larger array or string.

Let us elaborate on the steps of this algorithm -

• Initialize an empty unordered_map (hashmap) to track the frequency of characters in the current substring.

• Iterate over the characters in the string using a sliding window of size K.

• If a character is already in the hashmap, insert a new character until a unique character is obtained or the size of the sliding window is K.

• Move the sliding window one character and repeat the process until the end of the string is reached.

The time complexity of this algorithm is O(n), where n is the length of the string. This is because we iterate over each character in the string only once.

The Chinese translation of

Example

is:

Example

Let’s look at the C code that implements the above algorithm -

#include<bits/stdc++.h>
using namespace std;

string modifyString(string str, int K) {
   int n = str.size();
   string result = "";
   for(int i = 0; i < n; i++) {
      unordered_map<char, int> freq;
      int j = i;
      while(j < n && j < i + K) {
         while(j < n && freq[str[j]]) {
               result += 'a' + (rand() % 26); // insert a random character
         }
         freq[str[j++]]++;
         result += str[j];
      }
      i = j - 1;
   }
   return result;
}

int main() {
   string str = "abcabc";
   int K = 3;
   cout << modifyString(str, K) << endl;
   return 0;
}

Output

bcabc

When this code encounters repeated characters, it will randomly insert a lowercase English letter.

Test case example

Let us take an example to understand this problem better.

Consider the string str = "abcabc" and K = 3.

After running the code, you may get results similar to abcxyzabc. The three-character substrings are abc, bcx, cxy, xyz, yza, zab, abc, all of which contain unique characters.

Note− The results may vary because we are inserting random characters.

in conclusion

In summary, this algorithm provides a way to modify a string to ensure that each K-length substring has unique characters. This is an efficient solution that leverages the power of sliding window technology and the flexibility of C. We encourage you to experiment with different strings and K values ​​to fully grasp this concept.

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