1901D - Yet Another Monster Fight - CodeForces Solution

// To Harry Potter, Professor Dumbledore and JK Rowling

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

using lli = long long int;

int main() {
    lli N;
    scanf("%lld", &N);
    vector <lli> in(N + 1);
    vector <lli> pm(N + 1);
    vector <lli> sm(N + 1);
    vector <lli> an;
    lli mx;
    for (int i = 1; i <= N; i++) {
        cin >> in[i];
    }
    pm[1] = N - 1 + in[1];
    for (int j = 2; j <= N; j++) {
        pm[j] = max(pm[j - 1], N - j + in[j]);
    }
    sm[N] = N - 1 + in[N];
    for (int j = N - 1; j >= 1; j--) {
        sm[j] = max(sm[j + 1], j - 1 + in[j]);
    }
    for (int k = 1; k <= N; k++) {
        mx = in[k];
        if (k > 1) {
            mx = max(mx, pm[k - 1]);
        }
        if (k < N) {
            mx = max(mx, sm[k + 1]);
        }
        an.push_back(mx);
    }
    cout << (*min_element(an.begin(), an.end()));
    return 0;
}