Codeforces Round #543 (Div. 2, based on Technocup 2019 Final Round) B. Mike and Children(思维)

#include <bits/stdc++.h>
#define ll long long
#define maxn 1005
using namespace std;
int n,m;
int pre[maxn];
int vis[200005];

int main()
{
  memset(vis,0,sizeof(vis));
  scanf("%d",&n);
  for(int i=0;i<n;i++){
    scanf("%d",&pre[i]);
    vis[pre[i]] = 1;
  }
  int ans = 0;
  for(int i=0;i<=200000;i++){
    int cnt = 0;
    for(int j=0;j<n;j++){
      if(i < pre[j]) continue;
      if(vis[i-pre[j]] == 1)cnt ++;
    }
    ans = max(ans, cnt / 2);
  }
  printf("%d\n", ans);
  return 0;
}