Minimum changes to a string to make all substrings distinct

Given a string, find minimum number of changes to it so that all substrings of the string become distinct.

Examples :

Input :  str = "aab"
Output : 1
If we change one instance of 'a'
to any character from 'c' to 'z',
we get all distinct substrings.

Input : str = "aa"
Output : 1

To make all substrings distinct, every character must be different. So we simply need to count number of repeated characters. If length of string is more than 26, then we cannot convert it into a string with all distinct substrings (Here we assume that string should contain only lower case characters, ‘a’ to ‘z’)

Implementation:

C++

// CPP program to count number of changes
// to make all substrings distinct.
#include <bits/stdc++.h>
using namespace std;

const int MAX_CHAR = 26;

// Returns minimum changes to str so
// that no substring is repeated.
int minChanges(string &str)
{
    int n = str.length();

    // If length is more than maximum
    // allowed characters, we cannot
    // get the required string.
    if (n > MAX_CHAR)
       return -1;

    // Variable to store count of
    // distinct characters
    int dist_count = 0;

    // To store counts of different
    // characters
    int count[MAX_CHAR] = {0};

    for (int i = 0; i < n; i++) {
        if (count[str[i] - 'a'] == 0)
            dist_count++;
        count[(str[i] - 'a')]++;
    }

    // Answer is, n - number of distinct char
    return (n - dist_count);
}

// Driver function
int main()
{
    string str = "aebaecedabbee";
    cout << minChanges(str);
    return 0;
}

Java

// JAVA program to count number of changes
// to make all substrings distinct.
import java.lang.*;
import java.util.*;

class GFG
{
    static final int MAX_CHAR = 26;

    // Returns minimum changes to str so
    // that no substring is repeated.
    public static int minChanges(String str)
    {

        int n = str.length();

        // If length is more than maximum
        // allowed characters, we cannot
        // get the required string.
        if (n > MAX_CHAR)
            return -1;

        // Variable to store count of
        // distinct characters
        int dist_count = 0;
        int count[] = new int[MAX_CHAR];

        // To store counts of different
        // characters
        for(int i = 0; i < MAX_CHAR; i++)
            count[i] = 0;

        for (int i = 0; i < n; i++)
        {
            if(count[str.charAt(i)-'a'] == 0)
                dist_count++;
            count[str.charAt(i)-'a']++;
        }

        // Answer is, n - number of distinct char
        return (n-dist_count);
    }

    //Driver function
    public static void main (String[] args) {

        String str = "aebaecedabbee";

        System.out.println(minChanges(str));
    }
}

/* This code is contributed by Akash Singh*/

Python3

# Python3 program to count number of changes
# to make all substrings distinct.

MAX_CHAR = [26]

# Returns minimum changes to str so
# that no substring is repeated.
def minChanges(str):

    n = len(str )

    # If length is more than maximum
    # allowed characters, we cannot
    # get the required string.
    if (n > MAX_CHAR[0]):
        return -1

    # Variable to store count of
    # distinct characters
    dist_count = 0

    # To store counts of different
    # characters
    count = [0] * MAX_CHAR[0]

    for i in range(n):
        if (count[ord(str[i]) - ord('a')] == 0) :
            dist_count += 1
        count[(ord(str[i]) - ord('a'))] += 1

    # Answer is, n - number of distinct char
    return (n - dist_count)

# Driver Code
if __name__ == '__main__':
    str = "aebaecedabbee"
    print(minChanges(str))

# This code is contributed
# by SHUBHAMSINGH10

C#

// C# program to count number of changes
// to make all substrings distinct.
using System;

class GFG
{
    static int MAX_CHAR = 26;

    // Returns minimum changes to str so
    // that no substring is repeated.
    public static int minChanges(string str)
    {

        int n = str.Length;

        // If length is more than maximum
        // allowed characters, we cannot
        // get the required string.
        if (n > MAX_CHAR)
            return -1;

        // Variable to store count of
        // distinct characters
        int dist_count = 0;
        int []count = new int[MAX_CHAR];

        // To store counts of different
        // characters
        for(int i = 0; i < MAX_CHAR; i++)
            count[i] = 0;

        for (int i = 0; i < n; i++)
        {
            if(count[str[i] - 'a'] == 0)
                dist_count++;
            count[str[i] - 'a']++;
        }

        // Answer is, n - number of distinct char
        return (n - dist_count);
    }

    //Driver function
    public static void Main () {

        string str = "aebaecedabbee";

        Console.WriteLine(minChanges(str));
    }
}

/* This code is contributed by Akash Singh*/

PHP

<?php
// PHP program to count number of changes
// to make all substrings distinct.

// Returns minimum changes to str so
// that no substring is repeated.
function minChanges($str)
{
    $n = strlen($str);

    // If length is more than maximum
    // allowed characters, we cannot
    // get the required string.
    if ($n > 26)
    return -1;

    // Variable to store count of
    // distinct characters
    $dist_count = 0;

    // To store counts of different
    // characters
    $count = array_fill(0, 26, 0);
    for ($i = 0; $i < $n; $i++)
    {
        if ($count[ord($str[$i]) - 97] == 0)
            $dist_count++;
        $count[ord($str[$i]) -97]++;
    }

    // Answer is, n - number of
    // distinct char
    return ($n - $dist_count);
}

// Driver Code
$str = "aebaecedabbee";
echo minChanges($str);

// This code is contributed by mits
?>

Javascript

<script>

// javascript program to count number of changes
// to make all substrings distinct.

   var MAX_CHAR = 26;

    // Returns minimum changes to str so
    // that no substring is repeated.
    function minChanges(str)
    {

        var n = str.length;

        // If length is more than maximum
        // allowed characters, we cannot
        // get the required string.
        if (n > MAX_CHAR)
            return -1;

        // Variable to store count of
        // distinct characters
        var dist_count = 0;
        var count = Array.from({length: MAX_CHAR}, (_, i) => 0);

        // To store counts of different
        // characters
        for(var i = 0; i < MAX_CHAR; i++)
            count[i] = 0;

        for (var i = 0; i < n; i++)
        {
            if(count[str.charAt(i).charCodeAt(0)-'a'.charCodeAt(0)] == 0)
                dist_count++;
            count[str.charAt(i).charCodeAt(0)-'a'.charCodeAt(0)]++;
        }

        // Answer is, n - number of distinct char
        return (n-dist_count);
    }

// Driver function
var str = "aebaecedabbee";

document.write(minChanges(str));

// This code is contributed by Princi Singh
</script>

Output

8

Time complexity: O(n) where n is the length of given string

Auxiliary Space: O(1) it is using constant space for variables