Do you know what is called ``Coprime Sequence''? That is a sequence consists of
nnpositive integers, and the GCD (Greatest Common Divisor) of them is equal to 1.
``Coprime Sequence'' is easy to find because of its restriction. But we can try to maximize the GCD of these integers by removing exactly one integer. Now given a sequence, please maximize the GCD of its elements.
InputThe first line of the input contains an integer
T(1≤T≤10)T(1≤T≤10), denoting the number of test cases.
``Coprime Sequence'' is easy to find because of its restriction. But we can try to maximize the GCD of these integers by removing exactly one integer. Now given a sequence, please maximize the GCD of its elements.
In each test case, there is an integer n(3≤n≤100000)n(3≤n≤100000) in the first line, denoting the number of integers in the sequence.
Then the following line consists of nn integers a1,a2,...,an(1≤ai≤109)a1,a2,...,an(1≤ai≤109), denoting the elements in the sequence. OutputFor each test case, print a single line containing a single integer, denoting the maximum GCD. Sample Input
3 3 1 1 1 5 2 2 2 3 2 4 1 2 4 8Sample Output
1 2 2
这个题意呢,给你一系列质序列(就是他们的最大公约数为1)。然后呢,让你选一个数然后去掉,让他们的最大公约数变的最大。
思路:一看数据那么大,肯定不能直接算。但是又要将所有的组合都要算一遍,怎么办呢?用数组存头到尾存一遍他们的最大公约数,用pre[]代替好了,然后呢,怎样去一个数呢,把他“隔”过去好了,问题是怎样实现呢。
1.用·s[]逆着存一遍。
2、如果求去掉下标为i的数字后整个数列的gcd,直接将该数字前的所有数字的gcd(prefix[i-1])和该数字后所有数字的gcd(s[i+1])再求一下gcd就好了。
代码如下:
#include
#include
#include
using namespace std;
int a[100005],n;
int pre[100005],s[100005];
int gcd(int a,int b)
{
if(b==0)
return a;
else return gcd(b,a%b);
}
int main()
{
int t;
scanf("%d",&t);
while(t--)
{
scanf("%d",&n);
for(int i=0; i=0; i--)
{
s[i]=gcd(s[i+1],a[i]);
}
int ans=max(s[1],pre[n-2]);
for(int i=1; i