<p>Given a string <code>s</code>, return <em>the longest</em> <span data-keyword="palindromic-string"><em>palindromic</em></span> <span data-keyword="substring-nonempty"><em>substring</em></span> in <code>s</code>.</p>

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

<pre>
<strong>Input:</strong> s = &quot;babad&quot;
<strong>Output:</strong> &quot;bab&quot;
<strong>Explanation:</strong> &quot;aba&quot; is also a valid answer.
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;cbbd&quot;
<strong>Output:</strong> &quot;bb&quot;
</pre>

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

<ul>
	<li><code>1 &lt;= s.length &lt;= 1000</code></li>
	<li><code>s</code> consist of only digits and English letters.</li>
</ul>

class Solution {
public:
    string longestPalindrome(string s) {
        int n = s.length();
        string str = "#";
        for(int i = 0; i < n; i++) str = str + "" + s[i] + "#";
        n = str.length();
        vector<vector<bool>> v(n, vector<bool>(n));
        for(int i = 0; i < n; i++) v[i][i] = true;
        
        int mx = 0;
        int start = 0;
        int end = 0;
        for(int r = 0; r < n; r++){
            for(int l = 0; l < r; l++){
                if(str[l] == str[r] && v[l + 1][r - 1]){
                    if(r - l + 1 > mx){
                        mx = r - l + 1;
                        start = l;
                        end = r;
                    }
                    v[l][r] = true;
                }
            }
        }
        string res = "";
        for(int i = start; i <= end; i++){
            if(str[i] == '#') continue;
            res += str[i];
        }
        return res;
    }
};
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