2021-07-20 AcWing 3774. 亮灯时长

输入样例：

3
3 10
4 6 7
2 12
1 10
2 7
3 4

输出样例：

8
9
6

 

前缀和+贪心 

#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;

const int N = 100010;

int a[N], s1[N], s2[N];

int main()
{
    int T,n,m,res;
    scanf("%d", &T);
    while (T -- )
    {
        scanf("%d%d", &n, &m);
        for (int i = 1; i <= n; i ++ ) scanf("%d", &a[i]);
        a[ ++ n] = m;
        s1[n] = s2[n] = 0;
        for (int i = n - 1; i >= 0; i -- )
        {
            s1[i] = s1[i+1], s2[i] = s2[i+1];
            if (i%2 == 0) s1[i] += a[i+1]-a[i];
            else s2[i] += a[i+1] - a[i];
        }

        res=s1[0];
        for (int i = 0; i < n; i ++ )
        {
            int r = a[i+1]-a[i]-1;
            if (!r) continue;
            res = max(res, s1[0]-s1[i]+s2[i+1]+r);
        }
        printf("%d\n", res);
    }

    return 0;
}