Code : Yet Another Monster Fight

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

long long solve(vector<long long> &a) {
    int n = a.size();

    vector<long long> prefix_max_when_source_is_to_the_right(n);
    vector<long long> suffix_max_when_source_is_to_the_left(n);

    long long current_max = -1;
    for (int i = 0; i < n; i++) {
        // In the worst case, all elements to the right would be deleted first
        // Hence, x - right_count >= a[i]
        // x >= a[i] + right_count
        long long current_cost = a[i] + (n - 1) - i;
        prefix_max_when_source_is_to_the_right[i] =
            max(current_cost, current_max);
        current_max = max(current_max, current_cost);
    }

    current_max = -1;
    for (int i = n - 1; i >= 0; i--) {
        // In the worst case, all elements to the left would be deleted first
        // Hence, x - left_count >= a[i]
        // x >= a[i] + left_count
        long long current_cost = a[i] + i;
        suffix_max_when_source_is_to_the_left[i] =
            max(current_cost, current_max);
        current_max = max(current_max, current_cost);
    }

    vector<long long> source(n, 0);
    // source[i] is the answer when the source is the (i-th) element.
    for (int i = 0; i < n; i++) {
        source[i] = a[i];
        if (i - 1 >= 0) {
            source[i] =
                max(source[i], prefix_max_when_source_is_to_the_right[i - 1]);
        }
        if (i + 1 < n) {
            source[i] =
                max(source[i], suffix_max_when_source_is_to_the_left[i + 1]);
        }
    }
    return *min_element(source.begin(), source.end());
}

int main() {
    int n;
    cin >> n;

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

    auto res = solve(a);
    cout << res << endl;

    return 0;
}