Find And Replace in String — C++

// Time:  O(n + m), m is the number of targets
// Space: O(n)

class Solution {
public:
    string findReplaceString(string S, vector<int>& indexes, vector<string>& sources, vector<string>& targets) {
        vector<pair<int, string>> bucket(S.length());
        for (int i = 0; i < indexes.size(); ++i)  {
            if (S.compare(indexes[i], sources[i].length(), sources[i]) == 0) {
                bucket[indexes[i]] = {sources[i].length(), targets[i]};
            }
        }
        string result;
        for (int i = 0; i < S.length();) {
            if (bucket[i].first) {
                result += bucket[i].second;
                i += bucket[i].first;
            } else {
                result.push_back(S[i++]);
            }
        }
        return result;
    }
};