1940번 (silver 4)

https://www.acmicpc.net/problem/1940

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

using namespace std;
int main() {
	ios::sync_with_stdio(false);
	cin.tie(NULL);
	cout.tie(NULL);

	int N, num_sum, count = 0;
	cin >> N >> num_sum;
	vector<int> arr(N, 0);


	for (int i = 0; i < N; ++i) {
		cin >> arr[i];
	}
	sort(arr.begin(), arr.end());
	int start = 0, end = N - 1;
	while (end > start) {
		int sum = arr[start] + arr[end];
		if (sum > num_sum) {
			end--;
		}
		else if (sum < num_sum) {
			start++;
		}
		else {
			count++;
			start++;
			end--;
		}
	}

	cout << count;
	return 0;
}