Frontier

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

#define rep(i, a, b) for(int i = a; i < (b); i++)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

int main() {
    cin.tie(0)->sync_with_stdio(0);
    cin.exceptions(cin.failbit);

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

    vector<int> label(n, -1);
    {
        vector<bool> vis(s + 1);
        for (int i = 0; i + 2 < n; i++) {
            if (!vis[a[i + 1]]) {
                vis[a[i + 1]] = true;
                label[i] = 0; // start 
            }
        }
    }

    {
        vector<bool> vis(s + 1);
        for (int i = n - 2; i >= 0; i--) {
            if (!vis[a[i]]) {
                vis[a[i]] = true;
                label[i] = 1; // end 
            }
        }
    }

    for (int i = 0; i + 1 < n; i++) {
        if (label[i] == -1 && label[i + 1] == -1) {
            cout << "NO";
            return 0;
        }
    }
    cout << "YES";
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: WRONG ANSWER

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: WRONG ANSWER

Test 15

Verdict: WRONG ANSWER

Test 16

Verdict: WRONG ANSWER

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED