manacher算法，求回文串

用来求字符串最长回文串或者回文串的总数量

char a[N],c[N];
int radius[N];
int init()
{
	int n=strlen(a);
    for(int i=1,j=0;i<=2*n;j++,i+=2){
        c[i]='#';
        c[i+1]=a[j];
    }
    c[0]='$';
    c[2*n+1]='#';
    c[2*n+2]='@';
    c[2*n+3]='
';
    return 2*n+1;
}

ll manacher(){
    int mx=0,id=0,n=init();
    int maxn=0;
    ll ans=0;
    for(int i=1;i<=n;i++)
	{
        radius[i]=mx>i ? min(mx-i,radius[2*id-i]) : 1 ;
        while(c[i-radius[i]]==c[i+radius[i]]) radius[i]++;
        if(radius[i]+i>mx) mx=radius[i]+i,id=i;
        ans+=radius[i]/2;    
        maxn=max(maxn,radius[i]);
    }
    return ans;        //回文串数量
    return maxn;    //最长回文串
}
int main()
{
    scanf("%s",a);
    cout<<manacher();
    return 0;
}