双指针（对撞指针），模拟竞赛生的代码风格！

#include <iostream>
#include <algorithm>

using namespace std;

const int N = 510;
int a[N];

int main(void) {
  int n;
  scanf("%d", &n);
  for (int i = 1; i <= n; ++i) scanf("%d", &a[i]);

  sort(a + 1, a + n + 1);

  int ans = 0, l = 1, r = n;
  while (l < r) {
    int s = a[l] + a[r];
    if (!s) ++ans, ++l, --r;
    else if (s < 0) ++l;
    else --r;
  }

  printf("%d\n", ans);
}