翻转一段序列, 只能操作一次, 使得偶数索引的sum最大

import java.util.Scanner;

public class Main {
    static final int N = 200010;
    static int[] a = new int[N];
    static int n;

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        n = sc.nextInt();
        long sum = 0;
        for (int i = 1; i <= n; i++) {
            a[i] = sc.nextInt();
            if (i % 2 == 1) sum += a[i];
        }

        long dp = 0, max = 0;
        for (int i = 3; i <= n; i += 2) {
            long cur = a[i - 1] - a[i];
            dp = Math.max(dp + cur, cur);
            max= Math.max(max, dp);
        }
        dp = 0;
        for (int i = 1; i + 1 <= n; i += 2) {
            long cur = a[i + 1] - a[i];
            dp = Math.max(dp + cur, cur);
            max= Math.max(max, dp);
        }
        System.out.println(sum + max);
    }
}