题目来源：URAL 1748. The Most Complex Number

题意：求一个小于等于n的因子最多的数

思路：搜索+剪枝

#include <cstdio>
#include <cstring>
using namespace std;
typedef unsigned __int64 LL;
LL prime[16] = {2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53};
LL num, sum, n;

void dfs(int p, int q, LL x, LL y)
{
	if(p >= 16)
		return;
	if(x > n)
		return;
	if(y > sum)
	{
		num = x;
		sum = y;
	}
	if(y == sum && num > x)
		num = x;
	for(int i = 1; i <= q; i++)
	{
		double tmp = (double)x;
		if(tmp*prime[p] > n)
			break;
		dfs(p+1, i, x *= prime[p], y*(1+i));
	}	
}
int main()
{
	int T;
	scanf("%d", &T);
	while(T--)
	{
		scanf("%I64u", &n);
		sum = 0;
		dfs(0, 60, 1, 1);
		printf("%I64u %I64u\n", num, sum);
	}
	return 0;
}

URAL 1748. The Most Complex Number 反素数