LeetCode 249. 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"]
]

#include <string>
#include <unordered_map>
#include <vector>
#include <algorithm>
#include <iostream>
using namespace std;

vector< vector<string> > groupStrings(vector<string>& strings) {
  unordered_map<string, vector<string> > map;
  vector< vector< string> > res;
  for(auto str : strings) {
    string tmp = str;
    for(int i = tmp.size() - 1; i >= 0; --i) {
      tmp[i] = ('a' + (tmp[i] - tmp[0] + 26) % 26); // since it is group shifted.
    }
    map[tmp].push_back(str);
  }
  for(auto val : map) {
    sort(val.second.begin(), val.second.end());
    res.push_back(val.second);
  }
  return res;
}

int main(void) {
  vector<string> str{"abc", "bcd", "acef", "xyz", "az", "ba", "a", "z"};
  vector< vector<string> > res = groupStrings(str);
  for(int i = 0; i < res.size(); ++i) {
    for(int j = 0; j < res[i].size(); ++j) {
      cout << res[i][j] << " ";
    }
    cout << endl;
  }
}