First, find the maximum value of \(A_1, A_2, \ldots, A_N\).
Once you find the maximum value, which we denote by \(M\), all that left is to find the maximum value among \(A_1, A_2, \ldots, A_N\) that are not equal to \(M\).

Sample code

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

int main() {
	int n;
	cin >> n;
	vector<int> a(n);
	for (int i = 0; i < n; i++) cin >> a[i];
	int m = *max_element(a.begin(), a.end());
	int ans = 0;
	for (int e : a) if (e != m) ans = max(ans, e);
	cout << ans << endl;
}