893. Groups of Special Equivalent Strings

The basic idea is putting the signature of each string to a set, then the size of set is the number of groups. Since each string only contains lowercase letters, we can use two arrays odd[26] and even[26] to represents the frequency of 26 lowercase letters on odd place and even place of a string repectively. The signature of a string is apparently Arrays.toString(odd) + Arrays.toString(even). The code is as following:

public int numSpecialEquivGroups(String[] A) {
    Set<String> group = new HashSet<String>();
    for(int i = 0; i < A.length; i++) 
	group.add(getSignature(A[i]));
    return group.size();
}

public String getSignature(String s) {
    int odd[] = new int[26];
    int even[] = new int[26];
    for(int i = 0; i < s.length(); i += 2) 
	even[s.charAt(i) - 'a']++;
    for(int i = 1; i < s.length(); i += 2)
	odd[s.charAt(i) - 'a']++;
    StringBuilder sb = new StringBuilder("");
    for(int i = 0; i < 26; i++)
	sb.append(odd[i]);
    for(int i = 0; i < 26; i++)
	sb.append(even[i]);
    return sb.toString();
}