LeetCode 249. Group Shifted Strings

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

题目：

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"], 
A solution is:

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

题解：

维护一个HashMap, key是每个string 的 base型.

Note: char c, c - diff, need cast.

Time Complexity: O(nm), n = strings.length. m = average length of strings.

Space: O(hm.size()), HashMap size.

AC Java:

class Solution {
    public List<List<String>> groupStrings(String[] strings) {
        List<List<String>> res = new ArrayList<>();
        if(strings == null || strings.length == 0){
            return res;
        }
        
        Map<String, List<String>> hm = new HashMap<>();
        for(String s : strings){
            String base = getBase(s);
            hm.putIfAbsent(base, new ArrayList<String>());
            hm.get(base).add(s);
        }
        
        return new ArrayList<List<String>>(hm.values());
    }
    
    private String getBase(String s){
        if(s == null || s.length() == 0){
            return s;
        }
        
        char [] arr = s.toCharArray();
        int diff = arr[0] - 'a';
        for(int i = 0; i<arr.length; i++){
            arr[i] = (char)(arr[i]-diff);
            if(arr[i] < 'a'){
                arr[i] += 26;
            }
        }
        
        return new String(arr);
    }
}

类似Group Anagrams, Shifting Letters.