Odd String Difference — C++

// Time:  O(m * n), m is the number of words, n is the length of each word
// Space: O(1)

// freq table
class Solution {
public:
    string oddString(vector<string>& words) {
        for (int i = 0; i < size(words[0]) - 1; ++i) {
            unordered_map<int, vector<int>> lookup;
            for (int j = 0; j < size(words); ++j) {
                if (size(lookup[words[j][i + 1] - words[j][i]]) < 2) {
                    lookup[words[j][i + 1] - words[j][i]].emplace_back(j);
                }
            }
            if (size(lookup) == 2) {
                return size(cbegin(lookup)->second) == 1
                       ? words[cbegin(lookup)->second[0]]
                       : words[next(cbegin(lookup))->second[0]];
            }
        }
        return "";
    }
};