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;
}