Buddy Strings LeetCode solution in C++

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class Solution {
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 public:
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  // In case string is duplicated, check if string has any duplicate letters
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  bool dupeCase(string s) {
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    unordered_set<char> letters;
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    for (auto it : s) {
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      if (letters.count(it)) { // Found dupe letter (they can be swapped)
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        return true;
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      } else {
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        letters.insert(it);
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      }
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    }
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    return false; // all letters unique
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  }
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  bool buddyStrings(string s, string goal) {
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    // check len
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    if (goal.length() != s.length()) return false;
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    // If strings are the same, use diff. method
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    if (s == goal) return dupeCase(s);
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    // Track index of differences
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    vector<int> diff;
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    for (int i = 0; i < s.length(); i++) {
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      if (s[i] != goal[i]) {   // If diff found
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        if (diff.size() > 2) { // If this is third diff, return false
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          return false;
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        } else { // If not store in diff array
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          diff.push_back(i);
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        }
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      }
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    }
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    // If only one diff found, return false
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    if (diff.size() == 1) return false;
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    // else return if swap works
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    return (s[diff[0]] == goal[diff[1]] && s[diff[1]] == goal[diff[0]]);
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  }
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};