Frontier

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

#ifdef LOCAL
#define D(x) {x;}
#else
#define D(x)
#endif
#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()
using ll = long long;
using pii = pair<int, int>;
using vi = vector<int>;

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

    int n, s;
    cin >> n >> s;

    vector<vector<int>> id(s+1);
    for (int i = 0; i < n; ++i) {
        int x;
        cin >> x;
        id[x].push_back(i);
    }

    if (n == 1) {
        cout << "NO\n";
        return 0;
    }

    for (int i = 1; i <= s; ++i) {
        if (id[i].size() == 0) continue;
        int a = id[i][0];
        int b = id[i].back();
        for (int j = 1; j <= s; ++j) if (i != j) {
            auto it = lower_bound(all(id[j]), a);
            if (it != id[j].end()) {
                int c = *it;
                it = lower_bound(all(id[j]), b);
                if (it != id[j].begin()) {
                    --it;
                    int d = *it;
                    it = lower_bound(all(id[i]), c);
                    if (it != id[i].end()) {
                        if (c < *it && *it < d) {
                            cout << "NO\n";
                            return 0;
                        }
                    }
                }
            }
        }
        auto it = lower_bound(all(id[0]), a);
        if (it != id[0].end()) {
            if (a < *it && *it < b) {
                cout << "NO\n";
                return 0;
            }
        }
    }

    cout << "YES\n";
}

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: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED