Codeforces Round #334 (603A) Alternative Thinking [贪心]

time limit per test

2 seconds

memory limit per test

256 megabytes

input

standard input

output

standard output

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.

题意：

给出一个01串，先在你可以取反其中一个Substring（0变1,1变0），问这样操作后最长的0,1间隔子序列最长是多少。

解法：

显然，如果其中含有一个“11”或者“00”，最长序列实在原序列的基础上+1，例如10010 可以变成 10101，将右边的数字其以后的都取反即可。

然后，如果含有2个或更多的“11”或者“00”，其答案为原来的最长序列长度+2，例如1001011 可以变成 1010101，只需要把其中两个的靠近部分的那个数与中间所有数都取反即可。

至于求原来的最长串，就是相邻两个不同的数量，即间隔的0,1块的快数，例如（0000）（11111）（000）（1111），最长串就是4

代码：

略 ^  ^