codeforces131B

题意：给n个数，问这些数里相反数有几对。0与自己成相反数。

Input

5
-3 3 0 0 3

Output

3

Input

3
0 0 0

Output

3

Note

In the first sample the couples of opposite clients are: (1,2), (1,5) и (3,4).

In the second sample any couple of clients is opposite.

此题开始想用暴力枚举去做，然后果断超时了。

后来观察样例发现：当数值不为0时，ans=正数个数*负数个数，当数值为0时，ans=k*(k-1)/2。k为0的个数。由于数组的下标无法储存负数，所以需要用到STL的map。

#include <stdio.h>
#include<iostream>
#include<algorithm>
#include<map>
using namespace std;
typedef long long ll;
int n;
ll t;
map<int, ll>mp;
ll ans = 0;
int main()
{
	cin >> n;
	ll cnt = 0;
	for (int i = 1; i <= n; i++)
	{
		cin >> t;
		cnt = max(t, cnt);
		mp[t]++;
	}
	for (int i = 0; i <= cnt; i++) {
		if (i == 0)
			ans += (mp[i] * (mp[i] - 1)) / 2;
		else
			ans += mp[i] * (mp[-i]);
	}
	cout << ans << endl;
	return 0;
}