Codeforces Round #661 (Div. 3) C. Boats Competition

http://codeforces.com/problemset/problem/1399/C

双指针，枚举2-2*n。

#include
#include
#include
#include
typedef long long ll;
ll gcd(ll a, ll b)
{
	return b ? gcd(b, a % b) : a;
}
ll lcm(ll a, ll b) {
	return a* b / (gcd(a, b));
}
#define PII pair
using namespace std;
const int N = 1e5 + 5, mod = 1e9 + 7;
int qmi(int a, int k, int p)		//快速幂模板
{
	int res = 1;
	while (k)
	{
		if (k & 1) res = (ll)res * a % p;
		k >>= 1;
		a = (ll)a * a % p;
	}
	return res;
}
///////////////////////////////////////////////////////////
int a[N];
int main()
{
	int t;
	cin >> t;
	while (t--) {
		int n;
		cin >> n;
		for (int i = 1; i <= n; i++)
			cin >> a[i];
		sort(a + 1, a + 1 + n);
		int ans = 0;
		for (int i = 2; i <= 100; i++) {
			int cnt = 0;
			int l = 1; int r = n;
			while (l < r) {
				if (a[l] + a[r] == i) {
					cnt++;
					ans = max(ans, cnt);
					l++;
					r--;
				}
				else if (a[l] + a[r] > i) {
					r--;
				}
				else if (a[l] + a[r] < i) {
					l++;
				}
			}
		}
		cout << ans << endl;
	}
}
	

刚开始当成思维题写了，果然菜的真实