字符串-最长回文子串O(n)

#include<stdio.h>
#include<string.h>
#include<cmath>
#include<algorithm>
#include<iostream>
using namespace std;
const int MAXN=110010;


char s[2*MAXN];//MAXN太大只能放外面
int p[MAXN*2];
//求给定字符串str的最长回文子串
int solve(char str[])
{
    s[0]='$';
    s[1]='#';
    int n=strlen(str);
    for(int i=0;i<n;++i)
    {
        s[2*i+2]=str[i];
        s[2*i+3]='#';
    }
    n=n*2+2;
    s[n]=0;

    //-----------------------------------------

    int mx=0,id,ans=1;
    int mid=0;
    for(int i=0;i<n;++i)
    {
        if(mx>i) p[i]=min(p[2*id-i],mx-i);
        else p[i]=1;
        for(;s[i-p[i]]==s[i+p[i]];p[i]++);

        if(p[i]+i>mx)
        {
            mx=p[i]+i;
            id=i;
        }
        if(ans<p[i])
        {
            ans=p[i];
            mid=i;
        }
        //ans=max(ans,p[i]);
    }
    ans--;

    //if you need the string ~

//    mid=mid/2;
//    int f=mid-ans/2;
//    int t=mid+ans/2-1;
//    if(ans%2!=0) f--;
//    for(int i=f;i<=t;i++) cout<<str[i];
//    cout<<endl;

    return ans;
}



char str[MAXN];
int main()
{
    while(scanf("%s",str)!=EOF)
    {
        int ans=solve(str);
        cout<<ans<<endl;
    }
    return 0;
}