HDU 6025 - Coprime Sequence ( 前缀gcd+后缀gcd )

题意

给出一个数组, 删一个数使得剩余数字构成的gcd最大

思路

借鉴前缀和思路
求出前缀gcd+后缀gcd, 通过这两个数组来求最大gcd即可

AC代码

#include <iostream>
#include <algorithm>
#include <cstdio>
#include <cstring>
#define mst(a) memset(a, 0, sizeof(a))

using namespace std;

int gcd( int a, int b ){
    return b == 0 ? a : gcd(b, a%b);
}

const int maxn = 1e5+5;
int pre[maxn], suf[maxn];
int a[maxn];

int main()
{
    int T, n;
    scanf("%d",&T);
    while(T--){
        mst(pre);
        mst(suf);
        mst(a);
        scanf("%d",&n);
        for( int i = 1; i <= n; i++ )
            scanf("%d",&a[i]);
        pre[1] = a[1];
        for( int i = 2; i <= n; i++ )
            pre[i] = gcd(a[i], pre[i-1]);
        suf[n] = a[n];
        for( int i = n-1; i >= 1; i-- )
            suf[i] = gcd(a[i], suf[i+1]);
        int gg = max(pre[n-1], suf[2]);
        for( int i = 2; i <= n-1; i++ )
            gg = max(gg, gcd(pre[i-1], suf[i+1]));
        printf("%d\n",gg);
    }
    return 0;
}