Coprime Sequence

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 131072/131072 K (Java/Others)

Total Submission(s): 217    Accepted Submission(s): 126

Problem Description

Do you know what is called ``Coprime Sequence''? That is a sequence consists of n positive 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.

 

 

Input

The first line of the input contains an integer T(1T10), denoting the number of test cases.
In each test case, there is an integer n(3n100000) in the first line, denoting the number of integers in the sequence.
Then the following line consists of n integers a1,a2,...,an(1ai109), denoting the elements in the sequence.

 

Output

For 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 8

 

Sample Output

1

2

2

 

//T 组数据,n 个数,开始时,n 个数 gcd 为 1 ,现可以任意抽走一个数,问剩下数最大 gcd 是多少?

// st 表很好做,O(n*logn) + n * O(n) 的时间

 1 #include <iostream>
 2 #include <stdio.h>
 3 #include <string.h>
 4 using namespace std;
 5 #define MX 100005
 6 
 7 int n,m;
 8 int dp[MX][20];
 9 int mi[MX];
10 int data[MX];
11 
12 int gcd(int a,int b)
13 {return b==0?a:gcd(b,a%b);}
14 
15 void build()
16 {
17     mi[0]=-1;
18     for (int i=1;i<=n;i++)
19     {
20         mi[i]=((i&(i-1))==0) ? mi[i-1]+1:mi[i-1];   //注意位运算优先级很低
21         dp[i][0]=data[i];
22     }
23     for (int k=1;k<=mi[n];k++)
24     {
25         for (int i=1; (i+(1<<k)-1)<=n ;i++)
26         {
27             dp[i][k]=gcd(dp[i][k-1],dp[i+(1<<(k-1))][k-1]);
28         }
29     }
30 }
31 
32 int find_(int a,int b)
33 {
34     int x = mi[b-a+1];
35     return gcd(dp[a][x],dp[b-(1<<x)+1][x]);
36 }
37 
38 int main()
39 {
40     int T;
41     cin>>T;
42     while (T--)
43     {
44         scanf("%d",&n);
45         for (int i=1;i<=n;i++)
46             scanf("%d",&data[i]);
47         build();
48         int ans = 1;
49         for (int i=1;i<=n;i++)
50         {
51             int res;
52             if (i==1)
53                 res = find_(2,n);
54             else if (i==n)
55                 res = find_(1,n-1);
56             else
57                 res = gcd(find_(1,i-1),find_(i+1,n));
58             ans = max(ans,res);
59         }
60         printf("%d\n",ans);
61     }
62 }
View Code

 

 强行上log n 啊,啊哈哈,其实,只要从左向右扫一遍,存储gcd前缀和,再从右向左扫一遍,存gcd后缀和,然后枚举要剔除的数即可

 

posted @ 2017-05-08 19:51  happy_codes  阅读(302)  评论(0编辑  收藏  举报