class Solution {
 public:
  bool buddyStrings(string A, string B) {
    if (A.length() != B.length())
      return false;
    if (A == B && set<char>(A.begin(), A.end()).size() < A.length())
      return true;

    vector<int> diff;

    for (int i = 0; i < A.length(); ++i)
      if (A[i] != B[i])
        diff.push_back(i);

    return diff.size() == 2 && A[diff[0]] == B[diff[1]] &&
           A[diff[1]] == B[diff[0]];
  }
};