Find the count of palindromic sub-string of a string in its sorted form

Given a string str consisting of lowercase English alphabets, the task is to find the total number of palindromic sub-strings present in the sorted form of str.

Examples:

Input: str = “acbbd”
Output: 6
All palindromic sub-string in it’s sorted form (“abbcd”) are “a”, “b”, “b”, “bb”, “c” and “d”.

Input: str = “abbabdbd”
Output: 16

Naive approach: One way is to sort the given string and then count the total number of sub-strings present which are palindromes. For finding number of palindromic sub-strings this approach can be used which has time complexity of O(n^2).

Optimized approach: An efficient way is to count the frequency of each character and then for each frequency total number of palindromes will (n*(n+1))/2 as all the palindromic sub-strings of a sorted string will consist of the same character.
For example, palindromic sub-string for the string “aabbbcd” will be “a”, “aa”, …, “bbb”, “c”, … etc. Time complexity for this approach will be O(n).

  • Create a hash table for storing the frequencies of each character of the string str.
  • Traverse the hash table and for each non-zero frequency add (hash[i] * (hash[i]+1)) / 2 to the sum.
  • Print the sum in the end.

Below is the implementation of the above approach:

C++

filter_none

edit
close

play_arrow

link
brightness_4
code

// CPP program to find the count of palindromic sub-string
// of a string in it's ascending form
#include <bits/stdc++.h>
using namespace std;
  
const int MAX_CHAR = 26;
  
// function to return count of palindromic sub-string
int countPalindrome(string str)
{
    int n = str.size();
    int sum = 0;
  
    // calculate frequency
    int hashTable[MAX_CHAR];
    for (int i = 0; i < n; i++)
        hashTable[str[i] - 'a']++;
  
    // calculate count of palindromic sub-string
    for (int i = 0; i < 26; i++) {
        if (hashTable[i])
            sum += (hashTable[i] * (hashTable[i] + 1) / 2);
    }
  
    // return result
    return sum;
}
  
// driver program
int main()
{
    string str = "ananananddd";
  
    cout << countPalindrome(str);
    return 0;
}

chevron_right


Java

filter_none

edit
close

play_arrow

link
brightness_4
code

// Java program to find the count of palindromic sub-string 
// of a string in it's ascending form 
  
class GFG {
  
    final static int MAX_CHAR = 26;
  
// function to return count of palindromic sub-string 
    static int countPalindrome(String str) {
        int n = str.length();
        int sum = 0;
  
        // calculate frequency 
        int hashTable[] = new int[MAX_CHAR];
        for (int i = 0; i < n; i++) {
            hashTable[str.charAt(i) - 'a']++;
        }
  
        // calculate count of palindromic sub-string 
        for (int i = 0; i < 26; i++) {
            if (hashTable[i] != 0) {
                sum += (hashTable[i] * (hashTable[i] + 1) / 2);
            }
        }
  
        // return result 
        return sum;
    }
  
// driver program 
    public static void main(String[] args) {
        String str = "ananananddd";
  
        System.out.println(countPalindrome(str));
  
    }
}

chevron_right


Python3

filter_none

edit
close

play_arrow

link
brightness_4
code

# Python3 program to find the count of 
# palindromic sub-string of a string 
# in it's ascending form
MAX_CHAR = 26
  
# function to return count of 
# palindromic sub-string
def countPalindrome(str):
  
    n = len (str)
    sum = 0
  
    # calculate frequency
    hashTable = [0] * MAX_CHAR
    for i in range(n):
        hashTable[ord(str[i]) - 
                  ord('a')] += 1
  
    # calculate count of palindromic
    # sub-string
    for i in range(26) :
        if (hashTable[i]):
            sum += (hashTable[i] * 
                   (hashTable[i] + 1) // 2)
  
    # return result
    return sum
  
# Driver Code
if __name__ == "__main__":
  
    str = "ananananddd"
  
    print (countPalindrome(str))
  
# This code is contributed by ita_c

chevron_right


C#

filter_none

edit
close

play_arrow

link
brightness_4
code

// C# program to find the count of palindromic sub-string 
// of a string in it's ascending form 
using System;
                      
public class GFG{
   
    readonly static int MAX_CHAR = 26;
   
// function to return count of palindromic sub-string 
    static int countPalindrome(String str) {
        int n = str.Length;
        int sum = 0;
   
        // calculate frequency 
        int []hashTable = new int[MAX_CHAR];
        for (int i = 0; i < n; i++) {
            hashTable[str[i] - 'a']++;
        }
   
        // calculate count of palindromic sub-string 
        for (int i = 0; i < 26; i++) {
            if (hashTable[i] != 0) {
                sum += (hashTable[i] * (hashTable[i] + 1) / 2);
            }
        }
   
        // return result 
        return sum;
    }
   
// driver program 
    public static void Main() {
        String str = "ananananddd";
   
        Console.Write(countPalindrome(str));
   
    }
}
  
// This code is contributed by Rajput-Ji

chevron_right


PHP

Output:

26


My Personal Notes arrow_drop_up

Discovering ways to develop a plane for soaring career goals

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.



Improved By : Rajput-Ji, Ita_c, Mithun Kumar