题解

#include<iostream>
#include<cmath>
using namespace std;
int n,a[5010],Max;
int main(){
    cin>>n;
    for(int i=0;i<n;i++){
        int x;
        cin>>x;
        a[x]++;
    }
    for(int i=2;i<=10000;i++){
        int sum=0;
        for(int j=1;j<=5000;j++){
            int x=i-j;
            if(a[j]==0){
                continue;
            }
            if(x<1||x>5000){
                continue;
            }
            if(j>x){
                continue;
            }
            if(j==x){
                sum+=a[j]/2;
            }else{
                sum+=min(a[j],a[x]);
            }
        }
        Max=max(Max,2*sum);
    }
    cout<<Max;
    return 0;
}