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.

Circle shift.

Shift the string to a base state whose first letter is ‘a’. Use this base string as key, and the original string as value. Insert them into hashmap.

class Solution {
public:
    vector<vector<string>> groupStrings(vector<string>& strings) {
        vector<vector<string>> ans;
        unordered_map<string, vector<string>> map;
        for(auto it = strings.begin(); it != strings.end(); it++){
            string base = *it;
            if(base.size() == 0) continue;
            int offset = base[0] - 'a';
            for(int i = 0; i < base.size(); i++){
                base[i] = base[i] - offset;
                if(base[i] < 'a') base[i] += 26;
            }
            if(map.find(base) == map.end()){
                // new base string
                vector<string> element;
                element.push_back(*it);
                map[base] = element;
            }else{
                // append *it to base string vector
                map[base].push_back(*it);
            }
        }
        for(auto it = map.begin(); it != map.end(); it++){
            auto vstr = it->second;
            sort(vstr.begin(), vstr.end());
            ans.push_back(vstr);
        }
        return ans;
    }
};