<p>Strings <code>s1</code> and <code>s2</code> are <code>k</code><strong>-similar</strong> (for some non-negative integer <code>k</code>) if we can swap the positions of two letters in <code>s1</code> exactly <code>k</code> times so that the resulting string equals <code>s2</code>.</p>

<p>Given two anagrams <code>s1</code> and <code>s2</code>, return the smallest <code>k</code> for which <code>s1</code> and <code>s2</code> are <code>k</code><strong>-similar</strong>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s1 = &quot;ab&quot;, s2 = &quot;ba&quot;
<strong>Output:</strong> 1
<strong>Explanation:</strong> The two string are 1-similar because we can use one swap to change s1 to s2: &quot;ab&quot; --&gt; &quot;ba&quot;.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s1 = &quot;abc&quot;, s2 = &quot;bca&quot;
<strong>Output:</strong> 2
<strong>Explanation:</strong> The two strings are 2-similar because we can use two swaps to change s1 to s2: &quot;abc&quot; --&gt; &quot;bac&quot; --&gt; &quot;bca&quot;.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s1.length &lt;= 20</code></li>
	<li><code>s2.length == s1.length</code></li>
	<li><code>s1</code> and <code>s2</code> contain only lowercase letters from the set <code>{&#39;a&#39;, &#39;b&#39;, &#39;c&#39;, &#39;d&#39;, &#39;e&#39;, &#39;f&#39;}</code>.</li>
	<li><code>s2</code> is an anagram of <code>s1</code>.</li>
</ul>

class Solution {
public:
    string swap(string s, int i, int j){
        char temp = s[i];
        s[i] = s[j];
        s[j] = temp;
        return s;
    }

    vector<string> generateStrings(string s1, string s2){
        for (int i = 0; i < s1.size(); i++){
            if (s1[i] == s2[i]) continue;
            for (int j = 0; j < s1.size(); j++){
                if (s1[j] == s2[j]) continue;
                if (s1[i] == s2[j] && s1[j] == s2[i]) return {swap(s1, i, j)};
            }
        }

        vector<string> out;
        for (int i = 0; i < s1.size(); i++){
            if (s1[i] == s2[i]) continue;
            for (int j = i+1; j < s1.size(); j++){
                if (s1[j] == s2[j]) continue;
                if (s2[j] == s1[i]){
                    out.push_back(swap(s1, i, j));
                }
            }
        }
        return out;
    }

    int kSimilarity(string s1, string s2) {
        if (s1 == s2) return 0;
        int swaps = 0;
        set<string> seen;
        queue<string> q;
        q.push(s1);
        while (true){
            swaps++;
            int strings = q.size();
            for (int i = 0; i < strings; i++){
                vector<string> newStrings = generateStrings(q.front(), s2);
                q.pop();
                for (string s : newStrings){
                    if (s == s2) return swaps;
                    if (seen.find(s) != seen.end()) continue;
                    seen.insert(s);
                    q.push(s);
                }
            }
        }
        return -1;
    }
};
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