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Rearrange the string to maximize the number of palindromic substrings
  • Difficulty Level : Medium
  • Last Updated : 01 Apr, 2021

Given a string S consisting of lowercase characters(a-z) only, the task is to print a new string by rearranging the string in such a way that maximizes the number of palindromic substrings. In case of multiple answers, print any one. 
Note: even if some substrings coincide, count them as many times as they appear in the obtained string.
Examples:
 

Input: s = “aabab” 
Output: ababa 
string “ababa” has 9 palindromic substrings: “a”, “b”, “a”, “b”, “a”, “aba”, “bab”, “aba”, “ababa”.
Input: s = “aa” 
Output: aa 
The given string has the maximum number of palindromic substrings possible, “a”, “a” and “aa”. 
 

 

The problem might look to be a complex one but on solving for various cases and having observations will lead to an easy solution. 
A simple solution is to sort the string and print it. Sorting takes O(N * log N).
An efficient solution is to count the frequency of each character using a freq[] array and then construct the string using the freq[] array. 
Below is the implementation of the above approach.
 

C++




// C++ program to rearrange the
// string such to maximize the
// number of palindromic substrings
#include <bits/stdc++.h>
using namespace std;
 
// Function to return the newString
string newString(string s)
{
    // length of string
    int l = s.length();
 
    // hashing array
    int freq[26] = { 0 };
 
    // iterate and count
    for (int i = 0; i < l; i++) {
        freq[s[i] - 'a'] += 1;
    }
 
    // resulting string
    string ans = "";
 
    // form the resulting string
    for (int i = 0; i < 26; i++) {
 
        // number of times character appears
        for (int j = 0; j < freq[i]; j++) {
 
            // append to resulting string
            ans += (char)(97 + i);
        }
    }
 
    return ans;
}
 
// Driver Code
int main()
{
 
    string s = "aabab";
 
    cout << newString(s);
 
    return 0;
}

Java




// Java program to rearrange the
// string such to maximize the
// number of palindromic substrings
 
 
public class GFG {
     
    // Function to return the newString
    static String newString(String s)
    {
        // length of string
        int l = s.length();
 
        // hashing array
        int freq[] = new int [26] ;
 
        // iterate and count
        for (int i = 0; i < l; i++) {
            freq[s.charAt(i) - 'a'] += 1;
        }
 
        // resulting string
        String ans = "";
 
        // form the resulting string
        for (int i = 0; i < 26; i++) {
 
            // number of times character appears
            for (int j = 0; j < freq[i]; j++) {
 
                // append to resulting string
                ans += (char)(97 + i);
            }
        }
 
        return ans;
    }
 
 
    // Driver code
    public static void main(String args[])
    {
           String s = "aabab";
            System.out.println(newString(s));
    }
    // This Code is contributed by ANKITRAI1
}

Python3




# Python3 program to rearrange the
# string such to maximize the
# number of palindromic substrings
 
# Function to return the newString
def newString(s):
 
    # length of string
    l = len(s)
 
    # hashing array
    freq = [0] * (26)
 
    # iterate and count
    for i in range(0, l):
        freq[ord(s[i]) - ord('a')] += 1
 
    # resulting string
    ans = ""
 
    # form the resulting string
    for i in range(0, 26):
 
        # number of times character appears
        for j in range(0, freq[i]):
 
            # append to resulting string
            ans += chr(97 + i)
     
    return ans
 
# Driver Code
if __name__ == "__main__":
 
    s = "aabab"
    print(newString(s))
 
# This code is contributed by Rituraj Jain

C#




// C# program to rearrange the
// string such to maximize the
// number of palindromic substrings
using System;
class GFG
{
 
// Function to return the newString
static String newString(string s)
{
    // length of string
    int l = s.Length;
 
    // hashing array
    int[] freq = new int [26];
 
    // iterate and count
    for (int i = 0; i < l; i++)
    {
        freq[s[i] - 'a'] += 1;
    }
 
    // resulting string
    string ans = "";
 
    // form the resulting string
    for (int i = 0; i < 26; i++)
    {
 
        // number of times character appears
        for (int j = 0; j < freq[i]; j++)
        {
 
            // append to resulting string
            ans += (char)(97 + i);
        }
    }
 
    return ans;
}
 
 
// Driver code
public static void Main()
{
    string s = "aabab";
    Console.Write(newString(s));
}
}
 
// This code is contributed
// by ChitraNayal

Javascript




<script>
    // Javascript program to rearrange the
    // string such to maximize the
    // number of palindromic substrings
     
    // Function to return the newString
    function newString(s)
    {
        // length of string
        let l = s.length;
 
        // hashing array
        let freq = new Array(26);
        freq.fill(0);
 
        // iterate and count
        for (let i = 0; i < l; i++)
        {
            freq[s[i].charCodeAt(0) - 'a'.charCodeAt(0)] += 1;
        }
 
        // resulting string
        let ans = "";
 
        // form the resulting string
        for (let i = 0; i < 26; i++)
        {
 
            // number of times character appears
            for (let j = 0; j < freq[i]; j++)
            {
 
                // append to resulting string
                ans += String.fromCharCode(97 + i);
            }
        }
 
        return ans;
    }
 
     let s = "aabab";
    document.write(newString(s));
     
    // This code is contributed by divyeshrabadiya07.
</script>
Output: 



aaabb

 

Time Complexity: O(N) 
Auxiliary Space: O(26)
 

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