（寒假CF）惨淡一点

题解：类似于最大上升子序列

Iahub got bored, so he invented a game to be played on paper.

He writes n integers a₁, a₂, ..., a_n. Each of those integers can be either 0 or 1. He‘s allowed to do exactly one move: he chooses two indices i and j (1 ≤ i ≤ j ≤ n) and flips all values a_k for which their positions are in range [i, j] (that is i ≤ k ≤ j). Flip the value of xmeans to apply operation x = 1 - x.

The goal of the game is that after exactly one move to obtain the maximum number of ones. Write a program to solve the little game of Iahub.

The first line of the input contains an integer n (1 ≤ n ≤ 100). In the second line of the input there are n integers: a₁, a₂, ..., a_n. It is guaranteed that each of those n values is either 0 or 1.

Print an integer — the maximal number of 1s that can be obtained after exactly one move.

In the first case, flip the segment from 2 to 5 (i = 2, j = 5). That flip changes the sequence, it becomes: [1 1 1 0 1]. So, it contains four ones. There is no way to make the whole sequence equal to [1 1 1 1 1].

In the second case, flipping only the second and the third element (i = 2, j = 3) will turn all numbers into 1.

 1 #include<stdio.h>
 2 int main()
 3 {
 4     int n,i;
 5     int a[101];
 6     while(~scanf("%d",&n))
 7     {
 8         int sum=0;
 9         for(i=0;i<n;i++)
10         {
11             scanf("%d",&a[i]);
12             sum+=a[i];
13         }
14         int k=0,max=0;
15         for(i=0;i<n;i++)
16         {
17             if(a[i]==0)
18             {
19                 k++;
20             }
21             else
22             {
23                 if(k>0)
24                 k--;
25             }
26             if(k>=max)
27             max=k;
28         }
29         if(max)
30         printf("%d\n",max+sum);
31         else
32         printf("%d\n",max+sum-1);
33     }
34     return 0;
35 }