<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) {
        if (s.length() <= 1) {
            return s;
        }

        string longestStr = s.substr(0, 1);
        vector<vector<bool>> dp(s.length(), vector<bool>(s.length(), false));
        
        for (int i = 0; i < s.length(); i++) {
            dp[i][i] = true;
            for (int j = 0; j < i; j++) {
                // if string is 2 or 3 characters long and s[i] == s[j] we automatically know it's a palindrome
                if (s[j] == s[i] && (i - j <= 2 || dp[j + 1][i - 1])) {
                    dp[j][i] = true;
                    string temp = s.substr(j, i - j + 1);
                    longestStr = longestStr.size() < temp.size() ? temp : longestStr;
                }
            }
        }
        
        return longestStr;
    }
};
482630