k similar strings

/*
Strings A and B are K-similar (for some non-negative integer K) if we can swap the positions of two letters in A exactly K times so that 
the resulting string equals B.

Given two anagrams A and B, return the smallest K for which A and B are K-similar.

Example 1:

Input: A = "ab", B = "ba"
Output: 1
Example 2:

Input: A = "abc", B = "bca"
Output: 2
Example 3:

Input: A = "abac", B = "baca"
Output: 2
Example 4:

Input: A = "aabc", B = "abca"
Output: 2
Note:

1 <= A.length == B.length <= 20
A and B contain only lowercase letters from the set {'a', 'b', 'c', 'd', 'e', 'f'}
*/

class Solution {
public:
    int kSimilarity(string A, string B) {
        unordered_set<string> v;
        queue<string> q;
        q.push(A);
        v.insert(A);
        int step=0;
        while(q.size()){
            int size = q.size();
            while(size-- > 0){
                auto s = q.front();
                q.pop();
                if(s==B) return step;
                for(int i=0; i<s.size(); ++i){
                    if(s[i] == B[i]) continue;
                    for(int j=i+1; j<B.size(); ++j){
                        if(s[i] == B[j]){
                            swap(s[i], s[j]);
                            if(v.count(s) == 0){
                                q.push(s);
                                v.insert(s);
                            }
                                swap(s[i], s[j]);
                        }
                    }
                    break;
                }
            }
            ++step;
        }
        return step;
    }
};