传送门 You are given a string s[1…n] consisting of lowercase Latin letters. It is guaranteed that n=2k for some integer k≥0.
The string s[1…n] is called c-good if at least one of the following three conditions is satisfied:
The length of s is 1, and it consists of the character c (i.e. s1=c); The length of s is greater than 1, the first half of the string consists of only the character c (i.e. s1=s2=⋯=sn2=c) and the second half of the string (i.e. the string sn2+1sn2+2…sn) is a (c+1)-good string; The length of s is greater than 1, the second half of the string consists of only the character c (i.e. sn2+1=sn2+2=⋯=sn=c) and the first half of the string (i.e. the string s1s2…sn2) is a (c+1)-good string. For example: “aabc” is ‘a’-good, “ffgheeee” is ‘e’-good.
In one move, you can choose one index i from 1 to n and replace si with any lowercase Latin letter (any character from ‘a’ to ‘z’).
Your task is to find the minimum number of moves required to obtain an ‘a’-good string from s (i.e. c-good string for c= ‘a’). It is guaranteed that the answer always exists.
You have to answer t independent test cases.
Another example of an ‘a’-good string is as follows. Consider the string s=“cdbbaaaa”. It is an ‘a’-good string, because:
the second half of the string (“aaaa”) consists of only the character ‘a’; the first half of the string (“cdbb”) is ‘b’-good string, because: the second half of the string (“bb”) consists of only the character ‘b’; the first half of the string (“cd”) is ‘c’-good string, because: the first half of the string (“c”) consists of only the character ‘c’; the second half of the string (“d”) is ‘d’-good string. Input The first line of the input contains one integer t (1≤t≤2⋅104) — the number of test cases. Then t test cases follow.
The first line of the test case contains one integer n (1≤n≤131 072) — the length of s. It is guaranteed that n=2k for some integer k≥0. The second line of the test case contains the string s consisting of n lowercase Latin letters.
It is guaranteed that the sum of n does not exceed 2⋅105 (∑n≤2⋅105).
Output For each test case, print the answer — the minimum number of moves required to obtain an ‘a’-good string from s (i.e. c-good string with c= ‘a’). It is guaranteed that the answer exists.
输入 6 8 bbdcaaaa 8 asdfghjk 8 ceaaaabb 8 bbaaddcc 1 z 2 ac 输出 0 7 4 5 1 1
题意：额，这题的题意不太好描述，定义一个串为‘a’-good，当且仅当前n/2个字符为a且后半个串为‘b’good，或后n/2个字符为a且前半个串为‘b’good，依次往下推，具体还是应该看题…，你可以改动字符串中的字符为任意一个小写字母，问：最少操作多少次可以使串为‘a’-good;
思路：本来是想用贪心来着，但后来想了想不对，参考大佬的博客之后发现还是应该用搜索
