Luogu P4555 [国家集训队]最长双回文串

题目链接：​​传送门​​

要求的最长双回文串可以通过拼接两个回文串得到
所以我们处理出每个回文串的最左端和最右端
挨个拼接尝试
但是洛谷更新了数据之后好像会被hack掉…
所以，，，看代码吧

/**
 * @Date:   2019-03-10T18:56:14+08:00
 * @Last modified time: 2019-03-10T20:26:33+08:00
 */
#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <complex>
#include <algorithm>
#include <climits>
#include <queue>
#include <map>
#include <set>
#include <vector>
#include <iomanip>
#define
#define

using namespace std;
typedef long long ll;
char tmp[A >> 1], s[A];
int mx, id, p[A], n, ans, T, l[A], r[A];
void manacher() {
    for (int i = 1; i < n; i++) {
        if (mx > i) p[i] = min(p[id * 2 - i], mx - i);
        else p[i] = 1;
        while (s[i - p[i]] == s[i + p[i]]) ++p[i];
        if (i + p[i] > mx) {
            mx = p[i] + i;
            id = i;
        }
    }
    for (int i = 1, j = 1; i < n; i++)
        for (; j <= i + p[i] - 1; j++)
            l[j] = i;
    for (int i = n - 1, j = n - 1; i >= 1; i--)
        for (; j >= i - p[i] + 1; j--)
            r[j] = i;
    for (int i = 1; i < n; i++) ans = max(ans, r[i] - l[i]);
    if (ans == 3) puts("2");
    else if (ans == 5) puts("4");
    else printf("%d\n", ans);
}

int main(int argc, char const *argv[]) {
    scanf("%s", tmp); int lex = strlen(tmp);
    mx = -1, id = 0, n = 2, ans = 0; s[0] = '!'; s[1] = '#';
    for (int i = 0; i < lex; i++) s[n++] = tmp[i], s[n++] = '#';
    manacher();
}