CSES Solutions – Exponentiation II

Your task is to efficiently calculate values a^(b^c) modulo 10⁹+7.

Note that in this task we assume that 0⁰ = 1.

Examples:

Input: N = 2, queries[][] = {{3, 7, 1}, {2, 3, 2}}
Output:
2187
512
Explanation:

• 3 ^ (7 ^ 1) mod 10⁹+7 = 3 ^ 7 = 2187
• 2 ^ (3 ^ 2) mod 10⁹+7 = 2 ^ 9 = 512

Input: N = 3, queries[][] = {{3, 7, 1}, {15, 2, 2}, {3, 4, 5}}
Output:
2187
50625
763327764

Approach: To solve the problem, follow the below idea:

We can efficiently calculates a^b % mod using the binary exponentiation by squaring technique.

For any positive integer b, a^b can be calculated more efficiently by dividing b by 2 and squaring the result.

• If b is even, a^b = (a^(b/2)) ².
• If b is odd, a^b = a * (a^(b/2)) ².
• handling base cases where b is 0 or 1.

Modular Inverse Calculation: We’ll use Fermat’s Little Theorem, it states that if p is prime and a is not divisible by p, then a^(p-1) ≡ 1 (mod p). The modular inverse of a is a^(p-2) mod p.

Euler’s theorem: Since Euler theorem states that a^N ≡ a^NmodΦ(m) (mod M) so to calculate (a^(b^c)) modulo 10⁹+7 we will calculate ((b ^ c) modulo 10⁹ + 6). To get the final answer, we calculate (a ^ ((b ^ c) % 10⁹+6)) % 10⁹+7

Step-by-step algorithm:

• Calculate Modular Exponentiation:
  • If the exponent is 0, it returns 1. If the exponent is 1, it returns the base modulo the given value.
  • Recursively computes the result by dividing the exponent by 2 and squaring the result until the exponent is reduced to 0 or 1.
  • If the exponent is even, it squares the result. If the exponent is odd, it multiplies the result by the base after squaring.
• Calculate (b ^ c) % (10⁹+6) using modular exponentiation and store it in a variable, say power_bc.
• Calculate (a ^ power_bc) % (10⁹+7) using modular exponentiation and return it as the final answer.

Below is the implementation of the algorithm:

C++

#include <bits/stdc++.h>
#define ll long long
using namespace std;
const int MOD = 1000000007;

// Function to calculate power with modulo
ll powerWithModulo(ll base, ll exponent, int mod)
{
    if (exponent == 0)
        return 1;
    if (exponent == 1)
        return base % mod;

    // Recursive approach for exponentiation by squaring
    ll temp = powerWithModulo(base, exponent / 2, mod);

    if (exponent % 2 == 0)
        // If exponent is even, square the result
        return (temp * temp) % mod;
    else
        // If exponent is odd, multiply by base after
        // squaring
        return (((temp * temp) % mod) * base) % mod;
}

// Function to calculate modular inverse using Fermat's
// Little Theorem
ll calculateInverse(ll base, int mod)
{
    // Using Fermat's Little Theorem: a^(p-2) mod p
    return powerWithModulo(base, mod - 2, mod);
}

int main()
{
    ll N = 3;
    vector<vector<ll> > queries
        = { { 3, 7, 1 }, { 15, 2, 2 }, { 3, 4, 5 } };
    for (int i = 0; i < N; i++) {
        ll a = queries[i][0], b = queries[i][1],
           c = queries[i][2];
        // Calculate b^c modulo MOD-1
        ll power_bc = powerWithModulo(b, c, MOD - 1);

        // Calculate a^(b^c) modulo MOD
        ll result = powerWithModulo(a, power_bc, MOD);

        // Print the result for the current calculation
        cout << result << "\n";
    }

    return 0;
}

Java

import java.util.*;

public class Main {
    static final int MOD = 1000000007;
    // Function to calculate power with the modulo
    static long GFG(long base, long exponent, int mod) {
        if (exponent == 0)
            return 1;
        if (exponent == 1)
            return base % mod;
        // Recursive approach for the exponentiation by squaring
        long temp = GFG(base, exponent / 2, mod);
        if (exponent % 2 == 0)
            // If exponent is even
            // square the result
            return (temp * temp) % mod;
        else
            // If exponent is odd 
            // multiply by base after squaring
            return (((temp * temp) % mod) * base) % mod;
    }
    // Function to calculate modular inverse using the Fermat's Little Theorem
    static long calculateInverse(long base, int mod) {
        return GFG(base, mod - 2, mod);
    }
    public static void main(String[] args) {
        int N = 3;
        List<List<Long>> queries = Arrays.asList(
                Arrays.asList(3L, 7L, 1L),
                Arrays.asList(15L, 2L, 2L),
                Arrays.asList(3L, 4L, 5L)
        );
        for (int i = 0; i < N; i++) {
            long a = queries.get(i).get(0);
            long b = queries.get(i).get(1);
            long c = queries.get(i).get(2);
            // Calculate b^c modulo MOD-1
            long power_bc = GFG(b, c, MOD - 1);
            // Calculate a^(b^c) modulo MOD
            long result = GFG(a, power_bc, MOD);
            // Print the result for current calculation
            System.out.println(result);
        }
    }
}

Python3

MOD = 10**9 + 7

# Function to calculate power with modulo
def powerWithModulo(base, exponent, mod):
    if exponent == 0:
        return 1
    if exponent == 1:
        return base % mod

    # Recursive approach for exponentiation by squaring
    temp = powerWithModulo(base, exponent // 2, mod)

    if exponent % 2 == 0:
        # If exponent is even, square the result
        return (temp * temp) % mod
    else:
        # If exponent is odd, multiply by base after squaring
        return (((temp * temp) % mod) * base) % mod

# Function to calculate modular inverse using Fermat's Little Theorem
def calculateInverse(base, mod):
    # Using Fermat's Little Theorem: a^(p-2) mod p
    return powerWithModulo(base, mod - 2, mod)

if __name__ == "__main__":
    N = 3
    queries = [[3, 7, 1], [15, 2, 2], [3, 4, 5]]
    for query in queries:
        a, b, c = query
        # Calculate b^c modulo MOD-1
        power_bc = powerWithModulo(b, c, MOD - 1)

        # Calculate a^(b^c) modulo MOD
        result = powerWithModulo(a, power_bc, MOD)

        # Print the result for the current calculation
        print(result)

JavaScript

const MOD = BigInt(10**9 + 7);

// Function to calculate power with modulo
function powerWithModulo(base, exponent, mod) {
    if (exponent === 0n) {
        return 1n;
    }
    if (exponent === 1n) {
        return base % mod;
    }

    // Recursive approach for exponentiation by squaring
    let temp = powerWithModulo(base, exponent / 2n, mod);

    if (exponent % 2n === 0n) {
        // If exponent is even, square the result
        return (temp * temp) % mod;
    } else {
        // If exponent is odd, multiply by base after squaring
        return (((temp * temp) % mod) * base) % mod;
    }
}

// Function to calculate modular inverse using Fermat's Little Theorem
function calculateInverse(base, mod) {
    // Using Fermat's Little Theorem: a^(p-2) mod p
    return powerWithModulo(base, mod - 2n, mod);
}

// Main function
function main() {
    const N = 3;
    const queries = [[3, 7, 1], [15, 2, 2], [3, 4, 5]];
    for (let query of queries) {
        let [a, b, c] = query;
        // Calculate b^c modulo MOD-1
        let power_bc = powerWithModulo(BigInt(b), BigInt(c), MOD - 1n);

        // Calculate a^(b^c) modulo MOD
        let result = powerWithModulo(BigInt(a), power_bc, MOD);

        // Print the result for the current calculation
        console.log(result.toString());
    }
}

// Run the main function
main();

Output

2187
50625
763327764

Time complexity: O(N * log(exponent)), where N is the number of queries.
Auxiliary Space: O(N * log(exponent))