<p>Given a string <code>s</code> containing only three types of characters: <code>&#39;(&#39;</code>, <code>&#39;)&#39;</code> and <code>&#39;*&#39;</code>, return <code>true</code> <em>if</em> <code>s</code> <em>is <strong>valid</strong></em>.</p>

<p>The following rules define a <strong>valid</strong> string:</p>

<ul>
	<li>Any left parenthesis <code>&#39;(&#39;</code> must have a corresponding right parenthesis <code>&#39;)&#39;</code>.</li>
	<li>Any right parenthesis <code>&#39;)&#39;</code> must have a corresponding left parenthesis <code>&#39;(&#39;</code>.</li>
	<li>Left parenthesis <code>&#39;(&#39;</code> must go before the corresponding right parenthesis <code>&#39;)&#39;</code>.</li>
	<li><code>&#39;*&#39;</code> could be treated as a single right parenthesis <code>&#39;)&#39;</code> or a single left parenthesis <code>&#39;(&#39;</code> or an empty string <code>&quot;&quot;</code>.</li>
</ul>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<pre><strong>Input:</strong> s = "()"
<strong>Output:</strong> true
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> s = "(*)"
<strong>Output:</strong> true
</pre><p><strong class="example">Example 3:</strong></p>
<pre><strong>Input:</strong> s = "(*))"
<strong>Output:</strong> true
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 100</code></li>
	<li><code>s[i]</code> is <code>&#39;(&#39;</code>, <code>&#39;)&#39;</code> or <code>&#39;*&#39;</code>.</li>
</ul>

class Solution {
public:
    bool checkValidString(string s) {
        vector<vector<int>> memo(s.size(), vector<int>(s.size(), -1));
        return isValidString(0, 0, s, memo);
    }

    bool isValidString(int index, int openCount, const string& s, vector<vector<int>>& memo) {
        if (index == s.size()) return (openCount == 0);
        if (memo[index][openCount] != -1) return memo[index][openCount];

        bool isValid = false;
        if (s[index] == '*') {
            isValid |= isValidString(index+1, openCount+1, s, memo);
            if (openCount) isValid |= isValidString(index+1, openCount-1, s, memo);
            isValid |= isValidString(index+1, openCount, s, memo);
        } else if (s[index] == '(') isValid = isValidString(index+1, openCount+1, s, memo);
        else if (openCount) isValid = isValidString(index+1, openCount-1, s, memo);

        return memo[index][openCount] = isValid;
    }
};
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