Codeforces Round #276 (Div. 2)D - Maximum Value（筛法）

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<string>
#include<cmath>
#include<map>
#include<set>
#include<vector>
#include<algorithm>
#include<stack>
#include<queue>
#include<cctype>
#include<sstream>
using namespace std;
#define pii pair<int,int>
#define LL long long int
const int eps=1e-8;
const int INF=1000000000;
const int maxn=200009;
int n,a[maxn],ans=-1;
int main()
{
    //freopen("in9.txt","r",stdin);
    //freopen("out.txt","w",stdout);
    scanf("%d",&n);
    for(int i=0;i<n;i++)
    {
        scanf("%d",&a[i]);
    }
    if(n==1){printf("0\n");return 0;}
    sort(a,a+n);
    for(int i=0;i<n-1;i++)
    {
        if(i&&a[i]==a[i-1]) continue;
        for(int t=a[i]*2;t<=a[n-1];t+=a[i])
        {
            int p=lower_bound(a,a+n,t)-a;
            ans=max(ans,a[p-1]%a[i]);
        }
        ans=max(ans,a[n-1]%a[i]);
    }
    printf("%d\n",ans);
    //fclose(stdin);
    //fclose(stdout);
    return 0;
}