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NKPALIN.cpp
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NKPALIN.cpp
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#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> ii;
typedef unsigned long long ull;
#define X first
#define Y second
#define pb push_back
#define mp make_pair
#define ep emplace_back
#define EL printf("\n")
#define sz(A) (int) A.size()
#define FOR(i,l,r) for (int i=l;i<=r;i++)
#define FOD(i,r,l) for (int i=r;i>=l;i--)
#define fillchar(a,x) memset(a, x, sizeof (a))
#define faster ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
const int N = 2500;
int n, F[N][N];
string s, ans;
void solve(int i, int j, int L, int R) {
if (L > R) return ;
if (i == j) ans[L] = s[i];
else {
if (s[i] == s[j]) {
ans[L] = ans[R] = s[i];
solve(i+1, j-1, L+1, R-1);
} else {
if (F[i][j] == F[i+1][j])
solve(i+1, j, L, R);
else
solve(i, j-1, L, R);
}
}
}
int main() {
// freopen("INP.TXT", "r", stdin);
// freopen("OUT.TXT", "w", stdout);
getline(cin, s);
n = s.size()-1;
FOD(i,n,0)
FOR(j,i,n) {
if (i == j) F[i][j] = 1;
else {
if (s[i] == s[j]) {
if (i == j-1)
F[i][j] = 2;
else
F[i][j] = F[i+1][j-1] + 2;
} else
F[i][j] = max(F[i+1][j], F[i][j-1]);
}
}
ans.resize(F[0][n]);
solve(0, n, 0, F[0][n]-1);
cout << ans;
return 0;
}