palin.cpp

#include<iostream>
#include<algorithm>
#include<vector>

#define long long long
#define nln '\n'

using namespace std;

int main()
{
    string s1, s2;
    getline(cin, s1, nln);
    s2 = s1;
    reverse(s2.begin(), s2.end());



    long n = s1.size(), m = s2.size();
    s1 = ' ' + s1;
    s2 = ' ' + s2;

    vector<vector<long>> dp(n+1);
    for (auto &i : dp)
        i.resize(m+1, 0);

    for (long i = 1; i <= n; ++i)
        for (long j = 1; j <= m; ++j)
            if (s1[i] == s2[j])
                dp[i][j] = dp[i-1][j-1]+1;
            else
                dp[i][j] = max(dp[i-1][j], dp[i][j-1]);

    long i = n, j = m;
    string res = "";
    while (i > 0 && j > 0){
        if (s1[i] == s2[j]){
            res += s1[i];
            --i, --j;
        }
        else{
            if (dp[i-1][j] > dp[i][j-1])
                --i;
            else
                --j;
        }
    }
    cout << dp[n][m] << nln;
    reverse(res.begin(), res.end());
    cout << res << nln;
    return 0;
}