Longest Palindromic Substring

class Solution {
public:
    string longestPalindrome(string s) {
        vector<vector<bool>> f(s.size(), vector<bool>(s.size(), false));
        
        int maxLen = 1, start = 0;
        for(int i=0; i<s.size(); ++i)
        {
            f[i][i] = true;
            for(int j=0; j<i; ++j)
            {
                f[j][i] = (s[j] == s[i] && ((i-j<2)|| f[j+1][i-1]));
                if(f[j][i] && (i-j+1) > maxLen)
                {
                    start = j;
                    maxLen = i-j+1;
                }
            }
        }
        
        return s.substr(start, maxLen);
    }
};

class Solution {
public:
    string longestPalindrome(string s) {
        vector<char> tmp;
        tmp.push_back(‘_‘);
        for(int i=0; i<s.size(); ++i)
        {
            tmp.push_back(s[i]);
            tmp.push_back(‘_‘);
        }
        string ss = "";
        for(int i=0; i<tmp.size(); ++i)
            ss += tmp[i];

        int start = 0, maxLen = 1;
        for(int i=0; i<ss.size(); ++i)
        {
            int lt = i, rt = i;
            while(lt >= 0 && rt < ss.size() && ss[lt] == ss[rt])
            {
                --lt;
                ++rt;
            }
            
            if((rt - lt - 2) > maxLen)
            {
                start = lt + 1;
                maxLen = rt - lt - 1;
            }
        }
        
        tmp.clear();
        ss = ss.substr(start, maxLen);
        for(int i=0; i<ss.size(); ++i)
            if(ss[i] != ‘_‘)
                tmp.push_back(ss[i]);

        ss = "";
        for(int i=0; i<tmp.size(); ++i)
            ss += tmp[i];
        
        return ss;
    }
};