There exists a unique integer \(x\) such that there are only \(3\) cards with \(x\) written on it in the given pile. The answer is such \(x\).

For each \(k \ (1 \leq k \leq N)\), count the number of cards with \(k\) written on them, then the problem can be solved.

Sample code (C++) :

#include <bits/stdc++.h>
using namespace std;

int main() {
    int n;
    cin >> n;
    vector<int> count(n + 1);
    for (int i = 0; i < 4 * n - 1; ++i) {
        int a;
        cin >> a;
        count[a] += 1;
    }
    for (int i = 1; i <= n; ++i) {
        if (count[i] == 3) {
            cout << i << '\n';
        }
    }
    return 0;
}

Sample code (Python) :

n = int(input())
count = [0] * (n + 1)
for a in map(int, input().split()):
    count[a] += 1
for i in range(1, n + 1):
    if count[i] == 3:
        print(i)