二十分求助

#include <math.h>
#include <stdio.h>

#include <algorithm>
#include <cmath>
#include <cstdint>
#include <cstdlib>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <string>
#include <vector>
using namespace std;

int main(int argc, char* argv[]) {
    int n;
    cin >> n;
    int a[n];
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }
    sort(a, a + n - 1, [](int a, int b) -> bool { return a < b; });
    int s = 0;
    for (int i = 0; i < n; i++) {
        int sum = a[i], previous = -1;
        for (int j = 0; j < i; j++) {
            int n1 = a[j];
            if (n1 == previous) {
                continue;
            } else {
                previous = n1;
            }
            if (n1 > sum / 2) break;
            for (int k = j + 1; k < i; k++) {
                int n2 = a[k];
                if (n1 != n2 && n1 + n2 == sum) {
                    s++;
                }
            }
        }
    }
    cout << s << endl;
    return 0;
}