<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();
        if(n<=1) return s;
        int max_len = 1;
        string max_str = "";
        max_str+=s[0];
        string str = "#";
        for(int i=0;i<n;i++) {
            str = str + s[i] + '#';
        }
        n = str.length();
        vector<int> dp(n,0);
        int c = 0, r = 0;
        for(int i=0;i<n;i++) {
            if(i<r) {
                dp[i] = min(r-i, dp[2*c-i]);
            }
            while((i-dp[i]-1>=0) && (i+dp[i]+1<n) && (str[i-dp[i]-1]==str[i+dp[i]+1])) {
                dp[i]++;
            }
            if(i+dp[i] > r) {
                c = i;
                r = i+dp[i];
            }
            if(dp[i]>max_len) {
                max_len = dp[i];
                max_str = str.substr(i-dp[i], 2*dp[i]+1);
            }
        }
        string ans = "";
        for(int i=0;i<max_str.length();i++){
            if(max_str[i]!='#') {
                ans = ans + max_str[i];
            }
        }
        return ans;
    }
};
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