https://leetcode.com/problems/buddy-strings (C++)

buddy_strings.cpp

class Solution {
public:
    bool buddyStrings(const string& A, const string& B) {
        if (A.size() != B.size()) return false;
        vector<int> mismatches;
        bool has_duplicates = false;
        int letters = 0;
        for (int i = 0; i < A.size(); ++i) {
            int idx = 1 << (A[i] - 'a');
            has_duplicates |= letters & idx;
            letters |= idx;
            if (A[i] != B[i]) {
                if (mismatches.size() >= 2) return false;
                mismatches.push_back(i);
            }
        }
        if (mismatches.empty() && has_duplicates) return true;
        if (mismatches.size() < 2 || mismatches.size() > 2) return false;
        return A[mismatches[0]] == B[mismatches[1]] && A[mismatches[1]] == B[mismatches[0]];
    }
};