Largest sub-string where all the characters appear at least K times

Given a string str and an integer K, the task is to find the length of the longest sub-string S’ such that every character in S’ appears at least K times.

Input: s = “xyxyyz”, k = 2 
Output: 5 
“xyxyy” is the longest sub-string where 
every character appears at least twice.
Input: s = “geeksforgeeks”, k = 2 
Output: 2 

Approach: Consider all the possible sub-strings and for every sub-string, calculate the frequency of each of its character and check whether all the characters appear at least K times. For all such valid sub-strings, find the largest length possible.
Below is the implementation of the above approach: 

C++

// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;

#define MAX 26

// Function that return true if frequency
// of all the present characters is at least k
bool atLeastK(int freq[], int k)
{
    for (int i = 0; i < MAX; i++) {

        // If the character is present and
        // its frequency is less than k
        if (freq[i] != 0 && freq[i] < k)
            return false;
    }

    return true;
}

// Function to set every frequency to zero
void setZero(int freq[])
{
    for (int i = 0; i < MAX; i++)
        freq[i] = 0;
}

// Function to return the length of the longest
// sub-string such that it contains every
// character at least k times
int findlength(string str, int n, int k)
{

    // To store the required maximum length
    int maxLen = 0;

    int freq[MAX];

    // Starting index of the sub-string
    for (int i = 0; i < n; i++) {
        setZero(freq);

        // Ending index of the sub-string
        for (int j = i; j < n; j++) {
            freq[str[j] - 'a']++;

            // If the frequency of every character
            // of the current sub-string is at least k
            if (atLeastK(freq, k)) {

                // Update the maximum possible length
                maxLen = max(maxLen, j - i + 1);
            }
        }
    }

    return maxLen;
}

// Driver code
int main()
{
    string str = "xyxyyz";
    int n = str.length();
    int k = 2;

    cout << findlength(str, n, k);

    return 0;
}

Java

// Java Implementation of the above approach
class GFG
{

static final int MAX = 26;

// Function that return true if frequency
// of all the present characters is at least k
static boolean atLeastK(int freq[], int k)
{
    for (int i = 0; i < MAX; i++)
    {

        // If the character is present and
        // its frequency is less than k
        if (freq[i] != 0 && freq[i] < k)
            return false;
    }

    return true;
}

// Function to set every frequency to zero
static void setZero(int freq[])
{
    for (int i = 0; i < MAX; i++)
        freq[i] = 0;
}

// Function to return the length of the longest
// sub-string such that it contains every
// character at least k times
static int findlength(String str, int n, int k)
{

    // To store the required maximum length
    int maxLen = 0;

    int freq[] = new int[MAX];

    // Starting index of the sub-string
    for (int i = 0; i < n; i++)
    {
        setZero(freq);

        // Ending index of the sub-string
        for (int j = i; j < n; j++)
        {
            freq[str.charAt(j) - 'a']++;

            // If the frequency of every character
            // of the current sub-string is at least k
            if (atLeastK(freq, k))
            {

                // Update the maximum possible length
                maxLen = Math.max(maxLen, j - i + 1);
            }
        }
    }

    return maxLen;
}

// Driver code
public static void main(String args[])
{
    String str = "xyxyyz";
    int n = str.length();
    int k = 2;

    System.out.println(findlength(str, n, k));

}
}

// This code has been contributed by 29AjayKumar

Python3

# Python3 implementation of the approach
MAX = 26

# Function that return true if frequency
# of all the present characters is at least k
def atLeastK(freq, k) :

    for i in range(MAX) :

        # If the character is present and
        # its frequency is less than k
        if (freq[i] != 0 and freq[i] < k) :
            return False;

    return True;


# Function to set every frequency to zero
def setZero(freq) :

    for i in range(MAX) :
        freq[i] = 0;


# Function to return the length of the longest
# sub-string such that it contains every
# character at least k times
def findlength(string, n, k) :

    # To store the required maximum length
    maxLen = 0;

    freq = [0]*MAX;

    # Starting index of the sub-string
    for i in range(n) :
        setZero(freq);

        # Ending index of the sub-string
        for j in range(i,n) :
            freq[ord(string[j]) - ord('a')] += 1;

            # If the frequency of every character
            # of the current sub-string is at least k
            if (atLeastK(freq, k)) :

                # Update the maximum possible length
                maxLen = max(maxLen, j - i + 1);

    return maxLen;


# Driver code
if __name__ == "__main__" :

    string = "xyxyyz";
    n = len(string);
    k = 2;

    print(findlength(string, n, k));

# This code is contributed by AnkitRai01

C#

// C# Implementation of the above approach
using System;

class GFG
{

static int MAX = 26;

// Function that return true if frequency
// of all the present characters is at least k
static Boolean atLeastK(int []freq, int k)
{
    for (int i = 0; i < MAX; i++)
    {

        // If the character is present and
        // its frequency is less than k
        if (freq[i] != 0 && freq[i] < k)
            return false;
    }

    return true;
}

// Function to set every frequency to zero
static void setZero(int []freq)
{
    for (int i = 0; i < MAX; i++)
        freq[i] = 0;
}

// Function to return the length of the longest
// sub-string such that it contains every
// character at least k times
static int findlength(String str, int n, int k)
{

    // To store the required maximum length
    int maxLen = 0;

    int []freq = new int[MAX];

    // Starting index of the sub-string
    for (int i = 0; i < n; i++)
    {
        setZero(freq);

        // Ending index of the sub-string
        for (int j = i; j < n; j++)
        {
            freq[str[j] - 'a']++;

            // If the frequency of every character
            // of the current sub-string is at least k
            if (atLeastK(freq, k))
            {

                // Update the maximum possible length
                maxLen = Math.Max(maxLen, j - i + 1);
            }
        }
    }

    return maxLen;
}

// Driver code
public static void Main(String []args)
{
    String str = "xyxyyz";
    int n = str.Length;
    int k = 2;

    Console.WriteLine(findlength(str, n, k));
}
}

// This code is contributed by Princi Singh

Javascript

<script>

// Javascript implementation of the approach

var MAX = 26;

// Function that return true if frequency
// of all the present characters is at least k
function atLeastK(freq, k)
{
    for (var i = 0; i < MAX; i++) {

        // If the character is present and
        // its frequency is less than k
        if (freq[i] != 0 && freq[i] < k)
            return false;
    }

    return true;
}

// Function to set every frequency to zero
function setZero(freq)
{
    for (var i = 0; i < MAX; i++)
        freq[i] = 0;
}

// Function to return the length of the longest
// sub-string such that it contains every
// character at least k times
function findlength(str, n, k)
{

    // To store the required maximum length
    var maxLen = 0;

    var freq = Array(MAX).fill(0);

    // Starting index of the sub-string
    for (var i = 0; i < n; i++) {
        setZero(freq);

        // Ending index of the sub-string
        for (var j = i; j < n; j++) {
            freq[str[j].charCodeAt(0) - 'a'.charCodeAt(0)]++;

            // If the frequency of every character
            // of the current sub-string is at least k
            if (atLeastK(freq, k)) {

                // Update the maximum possible length
                maxLen = Math.max(maxLen, j - i + 1);
            }
        }
    }

    return maxLen;
}

// Driver code
var str = "xyxyyz";
var n = str.length;
var k = 2;
document.write( findlength(str, n, k));

</script>

Output:

5

Time Complexity: O(n²)
Auxiliary Space: O(MAX), where MAX is the maximum number of unique characters in the string.