<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 charMatch(char s, char p){
        if(s == '!'){
            return false;
        }
        if(p=='.'){
            return true;
        }
        return s==p;
    }
    int ind=0;
    bool isMatch(const string &s, const string &p, int si=0, int pi=0) {
        /*for(int i=0; i<ind; i++){
            cout << ' ';
        }
        cout << '|' << s.substr(0, si) << ' ' << p.substr(0, pi);
        cout << endl;
        ind++;
        struct deind{
            int*indp;
            deind(int*p){
                indp = p;
            }
            ~deind(){
                (*indp)--;
            }
        };
        deind bye(&ind); */
        if(si == s.size() && pi == p.size()){
            return true;
        }
         if(pi == p.size()){
            return false;
        }
        char sc = si == s.size() ? '!' : s[si];
        char pc = p[pi];
        bool star = pi+1 < p.size() && p[pi+1] == '*';

        if(star){
            if(isMatch(s, p, si, pi+2)){
                return true;
            }
            for(int sit=si; sit<s.size() && charMatch(s[sit], pc); sit++){
                if(isMatch(s, p, sit+1, pi+2)){
                    return true;
                }
            }
            return false;
        }else{
            if(charMatch(sc, pc)){
                return isMatch(s,p, si+1, pi+1);
            }else{
                return false;
            }
        }
    }
};
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