manacher 最长回文子串

确定当前已知能匹配到的最长处，看是否要更新最长

#include <bits/stdc++.h>
using namespace std;
const int N = 210005;
int p[N];
char str[N], s[N];
int main() {
	while (~scanf("%s", str)) {
		int n = strlen(str);
		for (int i = 0; i<n; i++) {
			s[2 * i] = '#';
			s[2 * i + 1] = str[i];
		}
		s[2 * n] = '#';
		s[2 * n + 1] = '\0';
		n = 2 * n + 1;
		int j = 0, mx = 0, id = 0;
		for (int i = 0; i < n; i++)p[i] = 0;
		for (int i = 0; i<n; i++) {
			if (i > mx)mx = i, id = i;
			j = id + id - i;
			if (j>=0&&j<n&&j - p[j]<=id + id - mx) {
				while (mx + 1 < n&&i + i - mx - 1 >= 0 && s[mx + 1] == s[i + i - mx - 1]) {
					mx++;
					p[i] = mx - i;
					id = i;
				}
			}
			else p[i] = p[j];
		}
		int ans = 0;
		for (int i = 0; i<n; i++)
			ans = max(ans, p[i]);
		printf("%d\n", ans);
	}
	return 0;
}