Codeforces 1304B. Longest Palindrome

根据数据范围，暴力可以解决，对每一个串，找与其互为回文的串，或者判断自身是否为回文串，然后两两将互为回文的串排列在头尾，中间放且只能最多放一个自身为回文串的串，因为题目说每个串都是不同的

#include<bits/stdc++.h>

using namespace std;

#define lowbit(x) ((x)&(-x))

typedef long long LL;

string str[];

int cache[], self[], s[], chosen[];

void run_case() {

    int n, m;

    cin >> n >> m;

    for(int i = ; i <= n; ++i)

        cin >> str[i];

    for(int i = ; i <= n; ++i) {

        bool flag = true;

        for(int j = ; j < m/ && flag; ++j) {

            if(str[i][j] != str[i][m-j-]) flag = false;

        }

        if(flag) {

            self[i] = ;

            continue;

        }

        if(cache[i]) continue;

        string now = str[i];

        reverse(now.begin(), now.end());

        for(int j = ; j <= n; ++j) {

            if(j != i && now == str[j]) {

                cache[i] = j; break;

            }

        }

    }

    vector<int> ans, selfs;

    int top = ;

    for(int i = ; i <= n; ++i)

        if(cache[i] && !chosen[i]) {

            ans.push_back(i);

            s[++top] = cache[i];

            chosen[cache[i]] = i;

        } else if(self[i]) selfs.push_back(i);

    if(selfs.size()) ans.push_back(selfs[]);

    while(top) {

        ans.push_back(s[top--]);

    }

    cout << ans.size() * m << "\n";

    for(auto i : ans)

        cout << str[i];

}

int main() {

    ios::sync_with_stdio(false), cin.tie();

    //cout.setf(ios_base::showpoint);cout.precision(10);

    //int t; cin >> t;

    //while(t--)

    run_case();

    cout.flush();

    return ;

}