878. Nth Magical Number - LeetCode

class Solution {
public:
    long long int ans, mmc, a, b, n;

    int gcd(int a, int b) {
        if (a == 0) return b;
        else return gcd(b % a, a);
    }

    int lcm(int a, int b) {
        return a*b/gcd(a, b);
    }

    void binary_search(long long int l, long long int r) {

        if (r>=l) {

            long long int mid = (r - l)/2 + l;

            long long int k = floor(mid/a) + floor(mid/b) - floor(mid/mmc);
            if (k >= n) {
                if (k == n){
                    ans = mid;
                }
                binary_search(l, mid-1);
            } else {
                binary_search(mid+1, r);
            }


        }


        return;
    }

    int nthMagicalNumber(int N, int A, int B) {
        ans = 1e18;
        mmc = lcm(A, B);
        a = A;
        b = B;
        n = N;
        long long int mod = 1e9 + 7;
        binary_search(1, 1e18);
        return ans % mod;
    }
};