<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) {
        ios_base::sync_with_stdio(0);
        cin.tie(0);
        cout.tie(0);
        int ans = 1;
        pair<int,int> ans_ind= {0,0};
        stack<pair<int,int>> st;
        for (int i = 0; i<s.length(); i++){
            st.push({i,i});
            if (i != s.length()-1 && s[i] == s[i+1]){
                st.push({i,i+1});
                ans = 2;
                ans_ind = {i,i+1};
            }

        }
        while(!st.empty()){
            auto [start,end] = st.top();
            // cout<<start<<" "<<end<<endl;
            st.pop();
            if (start == 0 || end == s.length()-1){
                continue;
            }
            else{
                if (s[start-1] == s[end+1]){
                    if (end-start+3 > ans){
                        ans = end-start+3;
                        ans_ind = {start-1,end+1};
                    }
                    st.push({start-1,end+1});
                }
            }
        }
        string ans_string = "";
        for (int i = ans_ind.first; i<= ans_ind.second; i++){
            ans_string += s[i];
        }
        return ans_string;
    }
};
47300