Longest Palindromic Substring
<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>
Two Pointers; String; Dynamic Programming
class Solution {
public:
    string longestPalindrome(string s) {
        int n = s.size();
        vector<vector<bool>> f(n, vector<bool>(n, true));
        int k = 0, mx = 1;
        for (int i = n - 2; ~i; --i) {
            for (int j = i + 1; j < n; ++j) {
                f[i][j] = false;
                if (s[i] == s[j]) {
                    f[i][j] = f[i + 1][j - 1];
                    if (f[i][j] && mx < j - i + 1) {
                        mx = j - i + 1;
                        k = i;
                    }
                }
            }
        }
        return s.substr(k, mx);
    }
};