Counting k-Length Strings with Character C Allowing Repeated Characters

Given a string S of length n containing distinct characters and a character C, the task is to count k-length strings that can be formed using characters from the string S, ensuring each string includes the specified character C, and characters from the given string S are used multiple times. Return the answer by taking the modulo of 1e9 + 7.

Note: There must be occurrence of given character in given string S.

Example:

Input: C = ‘a’, S = “abc”, k = 2
Output: 5
Explanation: All two-length strings are: {ab, ac, ba, bc, ca, cb, aa, bb, cc}
All valid strings including character 'C' are: {ab, ac, ba, ca, aa}

Input: C = 'd', S = "abcd", k = 3
Output: 61

Approach:

Think about complement approach that is: Count of total k length strings - Count of k length strings that doesn't containing given character C.

Formula: n^k - (n - 1)^k

• n^k: At every place of k length string have n option to fill the characters. So, k length string will have total n^k options.
• (n - 1)^k: We have assumed there doesn't exist any occurrence of given character 'C'. So, there would be only (n - 1) characters left and at every place of k length string have (n-1) options to fill the characters. So, k length string will have total (n - 1)^k options

Steps-by-step approach:

• Define constant M as 1e6 + 10 for the maximum size of the array.
• Define constant mod as 1e9 + 7 for the modulus in calculations.
• Binary Exponentiation Function (binaryExpo) for calculating power a^b:
  • Implement a function to calculate a^b under modulus mod using binary exponentiation.
  • Initialize a variable ans to 1.
  • Iterate through the binary representation of b.
  • If the current bit is 1, update ans with (ans * a) % mod.
  • Update a with (a * a) % mod and right-shift b.
  • Return the final value of ans.
• Problem-solving Function (solve):
  • Implement a function to solve the problem.
  • Calculate binaryExpo(n, k) and binaryExpo(n - 1, k).
  • Return the difference between the two results.

Below is the Implementation of the above approach:

#include <bits/stdc++.h>
using namespace std;
int const M
    = 1e6 + 10; // Define the maximum size of the array
long long const mod
    = 1e9 + 7; // Define the modulus for calculations

// Function to calculate a^b under modulus mod using binary
// exponentiation
long long binaryExpo(long long a, long long b)
{
    long long ans = 1;

    while (b) {
        if (b & 1) {
            ans = (ans * a) % mod;
        }
        a = (a * a) % mod;
        b >>= 1;
    }

    return ans;
}

// Function to solve the problem
int solve(int n, int k, char c, string& s)
{
    return binaryExpo(n, k) - binaryExpo(n - 1, k);
}

// Main function
int main()
{
    int n = 3; // Size of the string
    int k = 2; // Length of the strings to be formed
    char c = 'b'; // Character that must be included in the
                  // strings

    string s = "abc"; // Given string

    cout << solve(n, k, c, s); // Print the solution

    return 0;
}

import java.util.*;

public class Main {
    static final int M = 1000010; // Define the maximum size of the array
    static final long mod = 1000000007; // Define the modulus for calculations

    // Function to calculate a^b under modulus mod using binary exponentiation
    static long binaryExpo(long a, long b) {
        long ans = 1;

        while (b > 0) {
            if ((b & 1) == 1) {
                ans = (ans * a) % mod;
            }
            a = (a * a) % mod;
            b >>= 1;
        }

        return ans;
    }

    // Function to solve the problem
    static int solve(int n, int k, char c, String s) {
        return (int) ((binaryExpo(n, k) - binaryExpo(n - 1, k) + mod) % mod);
    }

    // Main function
    public static void main(String[] args) {
        int n = 3; // Size of the string
        int k = 2; // Length of the strings to be formed
        char c = 'b'; // Character that must be included in the strings

        String s = "abc"; // Given string

        System.out.println(solve(n, k, c, s)); // Print the solution
    }
}

# Define the maximum size of the array
M = 10**6 + 10

# Define the modulus for calculations
mod = 10**9 + 7

# Function to calculate a^b under modulus mod using binary exponentiation


def binaryExpo(a, b):
    ans = 1

    while b:
        if b & 1:
            ans = (ans * a) % mod
        a = (a * a) % mod
        b >>= 1

    return ans

# Function to solve the problem


def solve(n, k, c, s):
    return binaryExpo(n, k) - binaryExpo(n - 1, k)

# Main function


def main():
    n = 3  # Size of the string
    k = 2  # Length of the strings to be formed
    c = 'b'  # Character that must be included in the strings

    s = "abc"  # Given string

    print(solve(n, k, c, s))  # Print the solution


# Call the main function
main()

using System;

class Program {
    const int M
        = 1000000
          + 10; // Define the maximum size of the array
    const long Mod
        = 1000000007; // Define the modulus for calculations

    // Function to calculate a^b under modulus mod using
    // binary exponentiation
    static long BinaryExpo(long a, long b)
    {
        long ans = 1;

        while (b > 0) {
            if ((b & 1) == 1) {
                ans = (ans * a) % Mod;
            }
            a = (a * a) % Mod;
            b >>= 1;
        }

        return ans;
    }

    // Function to solve the problem
    static int Solve(int n, int k, char c, string s)
    {
        return (int)(BinaryExpo(n, k)
                     - BinaryExpo(n - 1, k));
    }

    // Main function
    static void Main(string[] args)
    {
        int n = 3; // Size of the string
        int k = 2; // Length of the strings to be formed
        char c = 'b'; // Character that must be included in
                      // the strings

        string s = "abc"; // Given string

        Console.WriteLine(
            Solve(n, k, c, s)); // Print the solution
    }
}
// This code calculates the difference between two values
// raised to the power of k under modulus Mod.

const M = 10**6 + 10; // Define the maximum size of the array
const mod = 10**9 + 7; // Define the modulus for calculations

// Function to calculate a^b under modulus mod using binary exponentiation
function binaryExpo(a, b) {
    let ans = 1;

    while (b) {
        if (b & 1) {
            ans = (ans * a) % mod;
        }
        a = (a * a) % mod;
        b >>= 1;
    }

    return ans;
}

// Function to solve the problem
function solve(n, k, c, s) {
    return binaryExpo(n, k) - binaryExpo(n - 1, k);
}

const n = 3; // Size of the string
const k = 2; // Length of the strings to be formed
const c = 'b'; // Character that must be included in the strings

const s = "abc"; // Given string

console.log(solve(n, k, c, s)); // Print the solution

5

Time Complexity: O(log(k))
Auxiliary Space: O(1)

Related Article: Counting K-Length Strings with Fixed Character in a Unique String (SET-1)