[洛谷P3805]【模板】manacher算法

题目大意：给你一个字符串，求出它的最长回文字段

题解：$manacher$算法

卡点：$p$数组未开两倍空间

C++ Code：

#include <cstdio>
#include <cstring>
#define maxn 11000010
char o[maxn], s[maxn << 1];
int p[maxn << 1];
int n;
inline int min(int a, int b) {return a < b ? a : b;}
inline int max(int a, int b) {return a > b ? a : b;}
int manacher(char *s) {
	int mx = 0, mid = 0, n = strlen(s), ans = 1;
	for (int i = 1; i < n; i++) {
		p[i] = mx > i ? min(p[(mid << 1) - i], mx - i) : 1;
		while (s[i - p[i]] == s[i + p[i]]) p[i]++;
		if (p[i] + i > mx) mx = p[i] + i, mid = i;
		ans = max(ans, p[i] - 1);
	}
	return ans;
}
int main() {
	scanf("%s", o);
	n = strlen(o);
	s[0] = '$';
	for (int i = 0; i < n; i++) {
		s[i << 1 | 1] = '$';
		s[i + 1 << 1] = o[i];
	}
	s[(n = n << 1 | 1)++] = '$';
	printf("%d\n", manacher(s));
	return 0;
}