双指针 + 排序

#include <algorithm>
#include <iostream>
using namespace std;

int main() {
    int n;
    cin >> n;

    int a[105];

    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }

    sort(a, a + n);

    int ans = 0;

    // 枚举作为结果的 a[k]
    for (int k = 0; k < n; k++) {
        int left = 0;
        int right = k - 1;

        while (left < right) {
            int sum = a[left] + a[right];

            if (sum == a[k]) {
                ans++;
                break;
            }

            if (sum < a[k]) {
                left++;
            } else {
                right--;
            }
        }
    }

    cout << ans << '\n';

    return 0;
}