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| #include <cstdio>
#include <algorithm>
using namespace std;
template <class Type>
void read (Type &x)
{
char c;
bool flag = false;
while ((c = getchar()) < '0' || c > '9')
c == '-' && (flag = true);
x = c - '0';
while ((c = getchar()) >= '0' && c <= '9')
x = (x << 1) + (x << 3) + c - '0';
flag && (x = ~x + 1);
}
template <class Type, class ...rest>
void read (Type &x, rest &...y) { read(x), read(y...); }
template <class Type>
void write (Type x)
{
x < 0 && (putchar('-'), x = ~x + 1);
x > 9 && (write(x / 10), 0);
putchar('0' + x % 10);
}
const int N = 5e5 + 10;
char str[N];
int n, cnt, last, f[N];
struct Node { int p, len, nxt[26]; } tr[N];
int get (int x, int t)
{
while (str[t - 1 - tr[x].len] != str[t]) x = tr[x].p;
return x;
}
int main ()
{
read(n);
scanf("%s", str + 1), str[0] = '$';
tr[0].p = 1, tr[1].len = -1;
last = 0, cnt = 2;
for (int i = 1; i <= n; ++i)
{
int c = str[i] - 'a', p = get(last, i);
if (!tr[p].nxt[c])
{
int q = cnt++;
tr[q].len = tr[p].len + 2;
tr[q].p = tr[get(tr[p].p, i)].nxt[c];
tr[p].nxt[c] = q;
if (tr[q].len <= 2) f[q] = tr[q].p;
else
{
int t = f[p];
while (str[i - 1 - tr[t].len] != str[i] || (tr[t].len + 2) * 2 > tr[q].len) t = tr[t].p;
f[q] = tr[t].nxt[c];
}
}
last = tr[p].nxt[c];
}
int res = 0;
for (int i = 2; i < cnt; ++i)
if (tr[f[i]].len * 2 == tr[i].len && tr[i].len % 4 == 0)
res = max(res, tr[i].len);
write(res);
return 0;
}
|