CodeForces - 604C Alternative Thinking （思维）

最新推荐文章于 2021-05-18 09:46:55 发布

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本文介绍了CodeForces上的一道编程题604C，内容涉及寻找字符串中最长交替子序列。在允许翻转一个连续子串的情况下，讨论如何最大化交替子序列的长度。通过分析，可以确定最佳翻转策略，以至少增加两个交替子序列的长度。

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题目链接：http://codeforces.com/problemset/problem/604/C点击打开链接

Kevin has just recevied his disappointing results on the USA Identification of Cows Olympiad (USAICO) in the form of a binary string of length n. Each character of Kevin's string represents Kevin's score on one of the n questions of the olympiad—'1' for a correctly identified cow and '0' otherwise.

However, all is not lost. Kevin is a big proponent of alternative thinking and believes that his score, instead of being the sum of his points, should be the length of the longest alternating subsequence of his string. Here, we define an alternating subsequence of a string as a not-necessarily contiguous subsequence where no two consecutive elements are equal. For example, {0, 1, 0, 1}, {1, 0, 1}, and {1, 0, 1, 0} are alternating sequences, while {1, 0, 0} and {0, 1, 0, 1, 1} are not.

Kevin, being the sneaky little puffball that he is, is willing to hack into the USAICO databases to improve his score. In order to be subtle, he decides that he will flip exactly one substring—that is, take a contiguous non-empty substring of his score and change all '0's in that substring to '1's and vice versa. After such an operation, Kevin wants to know the length of the longest possible alternating subsequence that his string could have.

Input

The first line contains the number of questions on the olympiad n (1 ≤ n ≤ 100 000).

The following line contains a binary string of length n representing Kevin's results on the USAICO.

Output

Output a single integer, the length of the longest possible alternating subsequence that Kevin can create in his string after flipping a single substring.

   Examples
 
     input
   
8
10000011

     output
   
5

     input
   
2
01

     output
   
2

Note

In the first sample, Kevin can flip the bolded substring '10000011' and turn his string into '10011011', which has an alternating subsequence of length 5: '10011011'.

In the second sample, Kevin can flip the entire string and still have the same score.

让你求数列的一个连续区间翻转之后最长的交替子序列

其实连续区间翻转起作用的只有头尾

而在相同子序列长度大于等于2的情况下无论怎么翻最优解肯定至少为原来的最长交替子序列上+2

#include <iostream>
#include <queue>
#include <stdio.h>
#include <stdlib.h>
#include <stack>
#include <limits>
#include <string>
#include <string.h>
#include <vector>
#include <set>
#include <map>
#include <algorithm>
#include <math.h>
using namespace std;
char a[111111];
int main()
{
    int n=0;
    scanf("%d",&n);
    getchar();
    int len=0;
    int cnt=1;
    scanf("%s",a);
    for(int i=0;i<n;i++)
    {
        if(i!=0)
        {
            if(a[i]!=a[i-1])
                cnt++;
            else
            {
                len++;
            }
        }
    }
        cout << cnt+min(len,2);
}