<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:
    int kSimilarity(string s1, string s2) {
        if(s1.size() == 1 && s2.size() == 1)
            return 0;
        
        if(s1[0] == s2[0])
            return kSimilarity(s1.substr(1), s2.substr(1));
        
        vector<int> match;
        match.clear();
        for(int i = 1; i < s1.size(); i++)
            if(s2[0] == s1[i] && s2[i] != s1[i]){
                if(s1[0] == s2[i]){ 
                    swap(s1[0], s1[i]);
                    return 1 + kSimilarity(s1.substr(1), s2.substr(1));
                }
                
                match.push_back(i);
            }
        
        int ans = 20;
        for(int i = 0; i < match.size(); i++){
            swap(s1[0], s1[match[i]]);
            ans = min(ans, 1 + kSimilarity(s1.substr(1), s2.substr(1)));
            swap(s1[0], s1[match[i]]);
        }
        
        return ans;
    }
};
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