Codeforces 484/B Maximum Value

Maximum Value

You are given a sequence a consisting of n integers. Find the maximum possible value of (integer remainder of a${}_i$ divided by a${}_j$), where 1 ≤ i, j ≤ n and a${}_i$ ≥ a${}_j$.

Input
The first line contains integer n — the length of the sequence (1 ≤ n ≤ 2·10${}^5$).

The second line contains n space-separated integers a i (1 ≤ a${}_i$ ≤ 10${}^6$).

Output
Print the answer to the problem.

Examples
inputCopy

3
3 4 5

outputCopy

2

二分出对于小于每个倍数的最大值

#include <bits/stdc++.h>
using namespace std;
const int maxn=2e5+7;
int a[maxn];
int main()
{
    int n;
    scanf("%d",&n);
    for(int i=1; i<=n; i++) scanf("%d",&a[i]);
    sort(a+1,a+1+n);
    int ans=0;
    for(int i=1; i<=n; i++)
    {
        if(i==1||a[i]!=a[i-1])
        {
            for(int j=2;j*a[i]<=a[n];j++)
            {
                int t=lower_bound(a+1,a+1+n,j*a[i])-a;
                ans=max(ans,a[t-1]%a[i]);
            }
            ans=max(ans,a[n]%a[i]);
        }
    }
    printf("%d\n",ans);
    return 0;
}