J - Parallelograms

There are n sticks, the i-th of which has length ai. Alex wants to assemble from them as many parallelograms as possible simultaneously, with each stick used at most in one parallelogram. What maximal number of parallelograms is it possible to assemble?
Input
The first line contains a single integer n (1 ≤ n ≤ 200000) — the number of sticks.

The second line contains n integers ai (1 ≤ ai ≤ 200000) — the lengths of sticks.
Output
Output a single integer — the maximal number of parallelograms that is possible to assemble.
Examples
Input
4
1 2 1 2
Output
1
Input
12
1 3 5 7 1 3 5 7 1 3 5 7
Output
2
思路：刚开始想把不同的边及数目都存放在数组中，之后先循环第一遍把所以边数>=4的都减去能组成菱形的边的个数，再循环第二遍判断是普通平行四边形的个数，结果wa了。后来改成了只统计每个边的个数，如果是奇数就剪掉以，因为组成平行四边形得用两条或四条。

#include<bits/stdc++.h>
using namespace std;
const int maxx=2e5+10;
int a[maxx];
int main()
{
    std::ios::sync_with_stdio(false);

    long long int n,x;
    cin>>n;
    memset(a,0,sizeof(a));
   for(int i=0;i<n;i++)
   {
       cin>>x;
       a[x]++;
   }
    long long int sum=0;
    for(int i=1; i<maxx; i++)
    {
        if(a[i]&&a[i]%2!=0)
            a[i]--;
        sum+=a[i];
    }
    cout<<sum/4<<endl;
    return 0;
}