牛客网暑期ACM多校训练营（第五场） G max

文章地址:http://henuly.top/?p=707

Description:

Give two positive integer c, n. You need to find a pair of integer (a,b) satisfy 1<=a,b<=n and the greatest common division of a and b is c.And you need to maximize the product of a and b

Input:

The first line has two positive integer c,n

Output:

Output the maximum product of a and b.

If there are no such a and b, just output -1

Sample Input:

2 4

Sample Output:

8

Hint:

1<=c,n<=10^9

题目链接

$gcd(i,j)=c(i,j\in[1,n])$，求$max(i*j)$。

$i=k1\times c,j=k2\times c$，$\because gcd(i,j)=c\therefore k1$和$k2$互质，找到最大的$k1,k2$即可。

$max(k)=\lfloor\frac{n}{c}\rfloor$，而$k-1$一定与$k$互质，$\therefore k1=\lfloor\frac{n}{c}\rfloor,k2=\lfloor\frac{n}{c}\rfloor-1$

注意特判$\lfloor\frac{n}{c}\rfloor=1$以及无解的情况

AC代码:

#include <bits/stdc++.h>
using namespace std;
#define mem(a,b) memset(a,b,sizeof(a))
#define pb push_back
#define mp make_pair
#define lowbit(x) (x&(-x))
#define XDebug(x) cout << #x << "=" << x << endl;
#define ArrayDebug(x,i) cout << #x << "[" << i << "]=" << x[i] << endl;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> PII;
typedef pair<double,double> PDD;
typedef pair<ll,ll> PLL;
const int INF = 0x3f3f3f3f;
const int maxn = 1e7 + 5;
const int mod = 1e9 + 7;
const double eps = 1e-8;
const double pi = asin(1.0) * 2;
const double e = 2.718281828459;
template <class T>
inline bool read(T &ret) {
    char c;
    int sgn;
    if (c = getchar(), c == EOF) {
        return 0;
    }
    while (c != '-' && (c < '0' || c > '9')) {
        c = getchar();
    }
    sgn = (c == '-') ? -1 : 1;
    ret = (c == '-') ? 0 : (c - '0');
    while (c = getchar(), c >= '0' && c <= '9') {
        ret = ret * 10 + (c - '0');
    }
    ret *= sgn;
    return 1;
}
template <class T>
inline void out(T x) {
    if (x > 9) {
        out(x / 10);
    }
    putchar(x % 10 + '0');
}

int main(int argc, char *argv[]) {
#ifndef ONLINE_JUDGE
    freopen("in.txt", "r", stdin);
    freopen("out.txt", "w", stdout);
#endif
    ll c, n;
    read(c); read(n);
    if (c > n) {
        printf("-1\n");
    }
    else if (n / c == 1) {
        printf("%lld\n", c * c);
    }
    else {
        printf("%lld\n", c * c * (n / c) * (n / c - 1));
    }
#ifndef ONLINE_JUDGE
    fclose(stdin);
    fclose(stdout);
    system("gedit out.txt");
#endif
    return 0;
}