<p>Given an input string <code>s</code>&nbsp;and a pattern <code>p</code>, implement regular expression matching with support for <code>&#39;.&#39;</code> and <code>&#39;*&#39;</code> where:</p>

<ul>
	<li><code>&#39;.&#39;</code> Matches any single character.​​​​</li>
	<li><code>&#39;*&#39;</code> Matches zero or more of the preceding element.</li>
</ul>

<p>The matching should cover the <strong>entire</strong> input string (not partial).</p>

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

<pre>
<strong>Input:</strong> s = &quot;aa&quot;, p = &quot;a&quot;
<strong>Output:</strong> false
<strong>Explanation:</strong> &quot;a&quot; does not match the entire string &quot;aa&quot;.
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;aa&quot;, p = &quot;a*&quot;
<strong>Output:</strong> true
<strong>Explanation:</strong> &#39;*&#39; means zero or more of the preceding element, &#39;a&#39;. Therefore, by repeating &#39;a&#39; once, it becomes &quot;aa&quot;.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;ab&quot;, p = &quot;.*&quot;
<strong>Output:</strong> true
<strong>Explanation:</strong> &quot;.*&quot; means &quot;zero or more (*) of any character (.)&quot;.
</pre>

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

<ul>
	<li><code>1 &lt;= s.length&nbsp;&lt;= 20</code></li>
	<li><code>1 &lt;= p.length&nbsp;&lt;= 20</code></li>
	<li><code>s</code> contains only lowercase English letters.</li>
	<li><code>p</code> contains only lowercase English letters, <code>&#39;.&#39;</code>, and&nbsp;<code>&#39;*&#39;</code>.</li>
	<li>It is guaranteed for each appearance of the character <code>&#39;*&#39;</code>, there will be a previous valid character to match.</li>
</ul>

class Solution {
public:
    bool match(string str, string pattern) {

        //std::cout << "match(" << str << "," << pattern << ")";  << std::endl;

        if( str.empty() ) {
            if( pattern.empty() ) {
                return true;
            }

            for( char c : pattern ) {
                if( (c >= 'a' && c <= 'z') || c == '.' ) {
                    return false;
                }
            }

            return true;
        }

        if( pattern.empty() ) {
            return false;
        }

        if( str.front() == pattern.front() || pattern.front() == '.' ) {
            return match(str.substr(1), pattern.substr(1));
        }

        bool frontStar = (pattern.front() >= 'A' && pattern.front() <= 'Z') || pattern.front() == '@';
        bool frontMatch = str.front() == (pattern.front() - ('A' - 'a')) || pattern.front() == '@';

        if( !frontStar ) {
            return false;
        }

        if( match(str, pattern.substr(1)) ) {
            return true;
        }
        else if( frontMatch ) {
            return match(str.substr(1), pattern.substr(1)) || match(str.substr(1), pattern);
        }

        return false;
    }

    bool isMatch(string s, string p) {
        size_t spos = 0;
        size_t ppos = 0;

        string pattern;
        for(size_t i =0; i < p.size(); i++) {
            if( i < p.size() -1 && p[i+1] == '*' ) {
                if( p[i] == '.' ) {
                    if( pattern.empty() || pattern.back() != '@') {
                        pattern.push_back('@');
                    }
                }
                else {
                    char symbol = p[i] + ('A' - 'a');

                    if( pattern.empty() || pattern.back() != symbol) {
                        pattern.push_back(symbol);
                    }
                }

                i++;
            }
            else {
                pattern.push_back(p[i]);
            }
        }

        return match(s, pattern);
    }
};
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