最长回文串O(N)算法 manacher

// 原串最大长度N
// 返回最大回文字串 res
#include<cstdio>

#include<cstring>

#include<cstdlib>

#include<string>

#include<algorithm>

using namespace std;

const int N = 1024;



int rid[N<<2];

string manacher(char *s){

    string t, res;    

    int L = strlen(s);    

    // init

    t += '?';

    for(int i = 0; i < L; i++)

        t += '#', t += s[i];

    t += "#*";

    // getrid

    for(int i=1,j=0,k,End=(int)t.size();i<End;){

        while( t[i-j-1]==t[i+j+1] ) j++;

        rid[i] = j;

        for(k=1;k<=j&&(rid[i-k]!=rid[i]-k);k++) 

            rid[i+k] = min( rid[i-k], rid[i]-k );

        i += k;

        j = max( 0,j-k );

    }    

    // Max

    int m = 0, pos;

    for(int i = 1; i < (int)t.size(); i++){

        if( m < rid[i]*2+(t[i]!='#') )

            m = rid[i]*2+(t[i]!='#'), pos = i;

    }

    for(int i = pos-rid[pos]; i <= pos+rid[pos]; i++){

        if( t[i] != '#' ) res += t[i];    

    }

    return res;

}





int main(){

    char s[N];

    scanf("%s", s);

    string ans = manacher(s); 

//    printf("MaxLength = %d\n", ans.size() );

    printf("%s\n", ans.c_str() );

    return 0;

}