【模板】manacher算法

传送门

某篇好文章

#include <cstdio>
#include <cstring>
#define N 22200000
#define min(x, y) ((x) < (y) ? (x) : (y))
#define max(x, y) ((x) > (y) ? (x) : (y))
 
int p[N];
char s[N];
int pos = 1, maxright, ans, n;
 
int main()
{
	int i, j;
	scanf("%s", s + 1);
	n = strlen(s + 1);
	s[0] = '$', s[n * 2 + 1] = '#';
	for(i = n; i >= 1; i--) s[i * 2] = s[i], s[i * 2 - 1] = '#';
	n = n * 2;
	for(i = 1; i <= n; i++)
	{
		p[i] = 1;
		if(i <= maxright)
			p[i] = min(p[pos * 2 - i], maxright - i + 1);
		while(s[i - p[i]] == s[i + p[i]]) p[i]++;
		if(maxright < i + p[i] - 1)
		{
			pos = i;
			maxright = i + p[i] - 1;
		}
		ans = max(ans, p[i] - 1);
	}
	printf("%d\n", ans);
	return 0;
}