Manacher（马拉车）

Manacher（马拉车）

 

#include<iostream>  
#include<string.h>
#include<algorithm>  
#include <cstdio>
using namespace std;
 
const int N  = 1e7 + 1e6 + 100;
char s[N];
char s_new[N * 2];
int p[N * 2];
 
int Init()
{
    int len = strlen(s);
    s_new[0] = '$';
    s_new[1] = '#';
    int j = 2;
 
    for (int i = 0; i < len; i++){
        s_new[j++] = s[i];
        s_new[j++] = '#';
    }
 
    s_new[j] = '\0';  //别忘了哦  
 
    return j;  //返回s_new的长度  
}
 
int Manacher()
{
    int len = Init();  //取得新字符串长度并完成向s_new的转换  
    int maxlen = -1;   //最长回文长度  
 
    int id;
    int mx = 0;
 
    for (int i = 1; i < len; i++){
        if (i < mx) p[i] = min(p[2 * id - i], mx - i);  //需搞清楚上面那张图含义, mx和2*id-i的含义
        else p[i] = 1;
         //不需边界判断，因为左有'$',右有'\0' 
        while (s_new[i - p[i]] == s_new[i + p[i]]) p[i]++;
 
        //我们每走一步i，都要和mx比较，我们希望mx尽可能的远，这样才能更有机会执行if (i < mx)这句代码，从而提高效率 
        if (mx < i + p[i])  {
            id = i;
            mx = i + p[i];
        }
 
       
    }
    for(int i = 0; i < len; ++i) { printf("%d ", i); } printf("\n");
    for(int i = 0; i < len; ++i) { printf("%d ", p[i]); } printf("\n");
    for(int i = 0; i < len; ++i) { printf("%c ", s_new[i]); } printf("\n");

    for(int i = 0; i < len; ++i){
        if(s_new[i] == '#') maxlen = max(maxlen, (p[i] - 1) / 2 * 2);
        else if(s_new[i] != '$') maxlen = max(maxlen, p[i] - 1);
    }
    return maxlen;
}
int main()
{
    scanf("%s", s);
    printf("%d\n", Manacher());
 
    return 0;
}