Parallelograms

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本文介绍了一个编程问题的解决方案，该问题是计算给定长度的棍子最多可以组成多少个平行四边形。通过使用C++编程语言，文章提供了一段代码示例，演示了如何对输入数据进行排序并计算出最大可能的平行四边形数量。

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J. Parallelograms

time limit per test

2.0 s

memory limit per test

256 MB

input

standard input

output

standard output

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
inputCopy
4
1 2 1 2
output
1
inputCopy
12
1 3 5 7 1 3 5 7 1 3 5 7
output
2

#include<iostream>
#include<algorithm>
#include<string.h>
using namespace std;
int a[200005];
int b[200005];
bool cmp(int a,int b)
{
    return a>b;
}
int main()
{
    int n;
    cin>>n;
    memset(b,0,sizeof(b));
    for(int i=0;i<n;i++)
    {
        cin>>a[i];
        b[a[i]]++;
    }
    sort(b,b+200000,cmp);
    int k=0;
    for(int i=0;i<=200000;i++)
    {
        if(b[i]>=2)
        {
            k+=b[i]/2;
        }
    }
    cout<<k/2<<endl;
    return 0;
}

题意就是给你n条边，让你计算出这些边最多能组成多少平行四边形：水过

AC 代码