link
快速乘板子
没什么要说的，上板子即可

#include <bits/stdc++.h>

using ll = long long;

auto main()->int {
    std::cin.tie(nullptr)->sync_with_stdio(false);
    
    auto qmul = [&](ll a, ll b, ll p) {
        ll ans = 0;
        for (; b; b >>= 1) {
            if (b & 1) {
                ans = (ans + a) % p;
            }
            a = (a + a) % p;
        }
        return ans;
    };

    ll A, B, P;
    std::cin >> A >> B >> P;
    std::cout << qmul(A, B, P);
    
    return 0;
}

5min $\textbf{\textcolor{#37A01D}{+}}$