<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:
    bool checkPalindrome(string s, int start, int end){
        while(start < end){
            if(s[start] != s[end]) return false;
            start++;
            end--;
        }
        return true;
    }

    string longestPalindrome(string s) {
        int n = (int) s.size();
        if(n == 1) return s;
        int arr[n];
        arr[0] = 0;
        for(int i=1; i<n; i++){
            if(arr[i-1]-1 >=0 && s[arr[i-1]-1] == s[i]){
                arr[i] = arr[i-1] -1;
                continue;
            }
            int index = arr[i-1];
            arr[i] = i;
            while(index < i){
                if(checkPalindrome(s, index, i)){
                    arr[i] = index;
                    break;
                }
                index++;
            }
        }
        string answer = "";
        for(int i=0; i<n; i++){
            string sub = s.substr(arr[i], i-arr[i]+1);
            if (sub.size() > answer.size()) answer = sub;
        }
        return answer;
    }
};
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