LeetCode Group Shifted Strings

原题链接在这里：https://leetcode.com/problems/group-shifted-strings/

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.

与Anagrams类似。存HashMap, key是base string.

Time Complexity: O(k*n), k = strings.length, n 是每个string的平均长度.

Space: O(k).

AC Java:

 1 public class Solution {
 2     public List<List<String>> groupStrings(String[] strings) {
 3         List<List<String>> res = new ArrayList<List<String>>();
 4         if(strings == null || strings.length == 0){
 5             return res;
 6         }
 7         
 8         HashMap<String, List<String>> hm = new HashMap<String, List<String>>();
 9         Arrays.sort(strings);
10         for(int i = 0; i<strings.length; i++){
11             String base = getBase(strings[i]);
12             if(!hm.containsKey(base)){
13                 List<String> item = new ArrayList<String>();
14                 item.add(strings[i]);
15                 hm.put(base, item);
16             }else{
17                 hm.get(base).add(strings[i]);
18             }
19         }
20         for(Map.Entry<String, List<String>> entry : hm.entrySet()){
21             res.add(entry.getValue());
22         }
23         return res;
24     }
25     
26     private String getBase(String str){
27         StringBuilder sb = new StringBuilder();
28         int diff = str.charAt(0) - 'a';
29         for(char c : str.toCharArray()){
30             int cDiff = c-'a'-diff;
31             if(cDiff < 0){
32                 cDiff+=26;
33             }
34             char newChar = (char)('a' + cDiff);
35             sb.append(newChar);
36         }
37         return sb.toString();
38     }
39 }