Count substrings made up of a single distinct character

Given a string S of length N, the task is to count the number of substrings made up of a single distinct character.
Note: For the repetitive occurrences of the same substring, count all repetitions.

Examples:

Input: str = “geeksforgeeks”
Output: 15
Explanation: All substrings made up of a single distinct character are {“g”, “e”, “ee”, “e”, “k”, “s”, “f”, “o”, “r”, “g”, “e”, “ee”, “e”, “k”, “s”}.

Input: str = “abaanndscx”
Output: 12

Naive Approach: The simplest approach to solve this problem is to generate all substrings from the given string count the number of substrings that consist of a single distinct character only.
Time Complexity: O(N³)
Auxiliary Space: O(1)

Effective Approach: Follow the steps below to optimize the above approach:

• Initialize a variable, say ans, to store the count of such substrings.
• Iterate over the characters of the string and check if the previous character, say pre, is the same as the current character or not.
• While the previous and current characters are found to be the same, increment subs and add to the answer.
• If the previous and current characters are found to be different, then reinitialize subs to 1.

Below is the implementation of the above approach:

C++

#include <iostream>
using namespace std;

  // Function to count the number
  // of substrings made up of a
  // single distinct character
  void countSubstrings(string s)
  {

    // Stores the required count
    int ans = 0;

    // Stores the count of substrings
    // possible by using current character
    int subs = 1;

    // Stores the previous character
    char pre = '0';

    // Traverse the string
    for (auto& i : s)
    {

      // If current character is same
      // as the previous character
      if(pre == i)
      {

        // Increase count of substrings
        // possible with current character
        subs += 1;
      }
      else
      {

        // Reset count  of substrings
        // possible with current character
        subs = 1;
      }

      // Update count of substrings
      ans += subs;

      // Update previous character
      pre = i;
    }
    cout << ans <<endl;
  }

// Driver code
int main()
{

    string s = "geeksforgeeks";
    countSubstrings(s);

    return 0;
}

// This code is contributed by splevel62.

Java

// Java program for the above approach
import java.io.*;
import java.util.*;
class GFG
{

  // Function to count the number
  // of substrings made up of a
  // single distinct character
  static void countSubstrings(String s)
  {

    // Stores the required count
    int ans = 0;

    // Stores the count of substrings
    // possible by using current character
    int subs = 1;

    // Stores the previous character
    char pre = '0';

    // Traverse the string
    for(char i : s.toCharArray())
    {

      // If current character is same
      // as the previous character
      if(pre == i)
      {

        // Increase count of substrings
        // possible with current character
        subs += 1;
      }
      else
      {

        // Reset count  of substrings
        // possible with current character
        subs = 1;
      }

      // Update count of substrings
      ans += subs;

      // Update previous character
      pre = i;
    }
    System.out.println(ans);
  }

// Driver Code
public static void main(String[] args)
{
    String s = "geeksforgeeks";
    countSubstrings(s);
}
}

// This code is contributed by souravghosh0416.

Python3

# Python3 Program to
# implement the above approach

# Function to count the number
# of substrings made up of a
# single distinct character


def countSubstrings(s):

    # Stores the required count
    ans = 0

    # Stores the count of substrings
    # possible by using current character
    subs = 1

    # Stores the previous character
    pre = ''

    # Traverse the string
    for i in s:

        # If current character is same
        # as the previous character
        if pre == i:

            # Increase count of substrings
            # possible with current character
            subs += 1
        else:

            # Reset count  of substrings
            # possible with current character
            subs = 1

        # Update count of substrings
        ans += subs

        # Update previous character
        pre = i

    print(ans)


# Driver Code
s = 'geeksforgeeks'
countSubstrings(s)

C#

// C# Program to
// implement the above approach
using System;
class GFG {

  // Function to count the number
  // of substrings made up of a
  // single distinct character
  static void countSubstrings(string s)
  {

    // Stores the required count
    int ans = 0;

    // Stores the count of substrings
    // possible by using current character
    int subs = 1;

    // Stores the previous character
    char pre = '0';

    // Traverse the string
    foreach(char i in s)
    {

      // If current character is same
      // as the previous character
      if(pre == i)
      {

        // Increase count of substrings
        // possible with current character
        subs += 1;
      }
      else
      {

        // Reset count  of substrings
        // possible with current character
        subs = 1;
      }

      // Update count of substrings
      ans += subs;

      // Update previous character
      pre = i;
    }
    Console.WriteLine(ans);
  }

  // Driver code
  static void Main() {
    string s = "geeksforgeeks";
    countSubstrings(s);
  }
}

// This code is contributed by divyesh072019.

Javascript

<script>
    // Javascript Program to implement
    // the above approach

    // Function to count the number
    // of substrings made up of a
    // single distinct character
    function countSubstrings(s)
    {

      // Stores the required count
      let ans = 0;

      // Stores the count of substrings
      // possible by using current character
      let subs = 1;

      // Stores the previous character
      let pre = '0';

      // Traverse the string
      for(let i = 0; i < s.length; i++)
      {

        // If current character is same
        // as the previous character
        if(pre == s[i])
        {

          // Increase count of substrings
          // possible with current character
          subs += 1;
        }
        else
        {

          // Reset count  of substrings
          // possible with current character
          subs = 1;
        }

        // Update count of substrings
        ans += subs;

        // Update previous character
        pre = s[i];
      }
      document.write(ans);
    }

    let s = "geeksforgeeks";
    countSubstrings(s);

</script>

Output:

15

Time Complexity: O(N)
Auxiliary Space : O(1)