Given a string, we can "shift" each of its letter to its successive letter, for example: "abc" -> "bcd". We can keep "shifting" which forms the sequence:

"abc" -> "bcd" -> ... -> "xyz"

Given a list of strings which contains only lowercase alphabets, group all strings that belong to the same shifting sequence.

For example, given: ["abc", "bcd", "acef", "xyz", "az", "ba", "a", "z"], 

Return:

[  ["abc","bcd","xyz"],  ["az","ba"],  ["acef"],  ["a","z"]]

Note: For the return value, each inner list's elements must follow the lexicographic order.

题解：

既然是group，那么我们需要有一个规则，把符合规则的放在一起。那么，规则是什么呢？

根据题目的意思，对于两串字符串，如果字符串的相邻字符的差值是一致的，那么我们就可以把它们放在一起。

但是对于az, ba这样的，字符之间的差值不一样啊。但是为何它们是一组的呢？

az字符之间的差值是25，ba之间字符的差值是-1，但是字符是每隔26就一循环，所以，对于-1，你一旦加上26就是25.

1 public class Solution { 2     public List
     
      
       > groupStrings(String[] strings) { 3         List
       
        
         > result = new ArrayList
         
          
           >(); 4 HashMap
           
            > map = new HashMap
            
             >(); 5 for (int i = 0; i < strings.length; i++) { 6 StringBuffer sb = new StringBuffer(); 7 for (int j = 0; j < strings[i].length(); j++) { 8 sb.append(Integer.toString(((strings[i].charAt(j) - strings[i].charAt(0)) + 26) % 26)); 9 sb.append(" ");10 }11 String shift = sb.toString();12 13 if (map.containsKey(shift)) {14 map.get(shift).add(strings[i]);15 } else {16 List
             
               list = new ArrayList
              
               ();17 list.add(strings[i]);18 map.put(shift, list);19 }20 }21 22 for (String s : map.keySet()) {23 Collections.sort(map.get(s));24 result.add(map.get(s));25 }26 return result;27 }28 }