正版题解

正版题解

#include <iostream>
#include <vector>
#include <unordered_set>

using namespace std;

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

    int count = 0;
    unordered_set<int> seen;
    seen.insert(a[0]);

    for (int i = 1; i < n; ++i) {
        int current = a[i];
        bool found = false;
        // 检查是否存在x, y, z在seen中使得x + y + z = current
        // 转换为是否存在x, y在seen中，使得current - x - y也在seen中
        for (int x : seen) {
            if (found) break;
            for (int y : seen) {
                int target = current - x - y;
                if (seen.find(target) != seen.end()) {
                    found = true;
                    break;
                }
            }
        }
        if (found) {
            count++;
        }
        seen.insert(current);
    }

    cout << count << endl;
    return 0;
}