最长回文子串O(n)

#include<vector>
#include<iostream>
#include<cstdio>
#include<string.h>
using namespace std;

const int N=3000010;
int n, p[N];
char s[N], str[N];

#define _min(x, y) ((x)<(y)?(x):(y))

void kp()
{
    int i;
    int mx = 0;
    int id;
    for(i=n; str[i]!=0; i++)
        str[i] = 0; 
    for(i=1; i<n; i++)
    {
        if( mx > i )
            p[i] = _min( p[2*id-i], p[id]+id-i );
        else
            p[i] = 1;
        for(; str[i+p[i]] == str[i-p[i]]; p[i]++)
            ;
        if( p[i] + i > mx )
        {
            mx = p[i] + i;
            id = i;
        }
    }
}

void init()
{
     int i, j, k;
     str[0] = '$';
     str[1] = '#';
     for(i=0; i<n; i++)
     {
      str[i*2+2] = s[i];
      str[i*2+3] = '#';
     }
     n = n*2+2;
     s[n] = 0;
}

int main()
{
    int i, ans;
    int t;
    cin >> t;
    while(t--)
    {
        cin >>s;
        n = strlen(s);
        init();
        kp();
        ans = 0;
        for(i=0; i<n; i++)
            if(p[i]>ans)
                ans = p[i];
        printf("%d\n", ans-1);
    }
    return 0;
}