Why Take the Modulus

Modulus is a very important concept in programming and computer science. Its function is to calculate the remainder after dividing two numbers. Modulus can be used to limit the value to a range and reduce the need for calculation. range, which can greatly reduce the amount of calculation and time.

Basic Modular Arithmetic

Through the Congruence Theorem, you can learn the modular operations of basic addition, subtraction, and multiplication.

$$ \text{If }\ A_0 \equiv A_1\ (mod\ m)\ \text{and }\ B_0 \equiv B_1\ (mod\ m) $$ $$ A_0 + B_0 \equiv A_1 + B_1\ (mod\ m) $$ $$ A_0 - B_0 \equiv A_1 - B_1\ (mod\ m) $$ $$ A_0 * B_0 \equiv A_1 * B_1\ (mod\ m) $$

However, in division, we cannot find the corresponding relationship through the above method. In division, we can convert it into multiplying a certain number by its reciprocal so that we can use modular arithmetic on it. We also call it modular multiplicative inverse.

Modular Multiplicative Inverse

$$ A^{-1} \equiv B\ (mod\ m) $$

Fermat’s Little Theorem

In Fermat’s Little Theorem, we are told that assuming $\ a\ $ is an integer and $\ p\ $ is a prime number, then $\ a^p - a\ $ is a multiple of $\ p\ $.

It can be expressed as $\ a^p \equiv a\ (mod\ p)\ $ , if $\ a\ $ is not a multiple of $\ p\ $ , it can be expressed as $\ a^{p-1} \equiv 1$.

In algorithm questions, a value $\ p\ $ is usually given, and we want to find $\ a^{-1}\ $, so $\ a^{p-1} \equiv 1\ $ on both sides Multiplying $\ a^{-1}\ $ is

$$ a^{p - 1} \equiv a^{-1} $$

Therefore the reciprocal of $\ a\ $ modulo p is $\ a^{p - 2}\ $. When $\ p\ $ is a prime number, finding $\ a^{-1}\ $ is equivalent to finding $\ a^{p - 2}\ $.

LeetCode Usage

In LeetCode, the permutation and combination formula is used to calculate how many results there are. In order to facilitate calculation, we usually calculate $\ n!\ $ and $\ {n!}^{-1}\ $ first, so how to calculate $\ { n!}^{-1}\ $ will require the application of Fermat’s Little Theorem and the modular reciprocal. Usually in algorithm questions, in order to reduce the number, the modulus ($\ 10^9 + 7\ $) is usually required. Therefore, after we calculate $\ n!\ $, we can use its value to calculate $\ {n! }^{10^9 + 7 - 2}\ $ to find the reciprocal of $\ {n!}^{-1} $ modulo $\ (10^9 +7)\ $.

Code

int const MOD = 1e9 + 7;
int exp(int a, int b) {
    int res = 1;
    while (b > 0) {
        if (b & 1)
            ans = 1LL * ans * a % MOD;
        a = 1LL * a * a % MOD;
        b >>= 1;
    }
    return ans;
}

int main() {
    int n = 10;
    vector<int> fact (n + 1, 1), invFact(n + 1, 1);
    for (int i = 1; i <= n; ++i) {
        finv[i] = 1LL * finv[i - 1] * i % MOD;
        invFact[i] = exp(finv[i], MOD - 2);
    }
}