Manacher最长回文子串

Posted 2021-12-24 thusloop

//#pragma GCC optimize(2)
//#pragma GCC optimize(3,"Ofast","inline")
#include<bits/stdc++.h>
#define int long long
#define ld long double
#define fi first
#define se second
#define pb push_back
#define pii pair<int,int>
#define ios ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
using namespace std;
const int inf=8e18;
const int maxn=2e5+100;
char A[maxn* 2],a[maxn];
int B[maxn * 2];
int manacher(char s[], int len)

	int l = 0;
	A[l++] = '$';   //0下标存储为其他字符
	A[l++] = '#';
	for (int i = 0; i < len; i++)
	
		A[l++] = s[i];
		A[l++] = '#';
	
	A[l] = 0;       //空字符
	int mx = 0;
	int id = 0;
	for (int i = 1; i < l; i++)
	
		B[i] = mx > i ? min(B[2 * id - i], mx - i) : 1;
		while (A[i + B[i]] == A[i - B[i]])
		
			B[i]++;
		
		if (i + B[i] > mx)
		
			mx = i + B[i];
			id = i;
		
	
	int ans = 0;
	for (int i = 0; i < 2 * len + 2; i++)
	
		ans = max(ans, B[i] - 1);
	
	return ans;

signed main()

	cin>>a;
	int n=strlen(a);
	cout<<manacher(a,n)<<"\\n";

/*
 * abaaba
 * i:   0  1  2  3  4  5  6  7  8  9 10 11 12 13 14
 * A:   $  #  a  #  b  #  a  #  a  #  b  #  a  # '\\0'
 * B:   1  1  2  1  4  1  2  7  2  1  4  1  2  1    //以第i个为中心的回文半径（包括第i个）
 */