[실3] 3273 - 두 수의 합

#include <iostream>
#include <algorithm>
#include <vector>

using namespace std;

int main()
{
	int n;
	cin >> n;
	vector<int> v(n);

	for (int i = 0; i < n; i++)
		cin >> v[i];

	sort(v.begin(), v.end());
	int x;
	cin >> x;

	int s = 0, e = n - 1, cnt = 0;

	while (s < e)
	{
		int a = v[s], b = v[e];
		int sum = a + b;

		if (sum == x)
			cnt++;

		if (sum < x)
			s++;

		else
			e--;
	}
	cout << cnt;
	return 0;
}