poj 3450 Corporate Identity（二分长度 + 暴力kmp）

题目：http://poj.org/problem?id=3450
题目大意：给你n个字符串，让你求出他们的最长公共子串，有多个时，输出字典序最小的，如果没有，就输出那句话。
和之前 poj 那道一样，暴力做，可是看复杂度好像很高的样子，poj 数据水了？。。。 = =

代码如下：

#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;

const int MAXN = 4004;

const int MAX_LEN = 222;

int fail[MAX_LEN];

void get_fail(char *s)
{
    int len = strlen(s);
    int j = -1;
    fail[0] = -1;
    for(int i = 1;i<len;i++)
    {
        while(j>=0 && s[i] != s[j+1])
            j = fail[j];
        if(s[i] == s[j+1]) j++;
        fail[i] = j;
    }
}

int kmp(char *t,char *p)
{
    int len_t = strlen(t);
    int len_p = strlen(p);
    get_fail(p);
    int j = -1;
    for(int i = 0;i<len_t;i++)
    {
        while(j >= 0 && t[i] != p[j+1])
            j = fail[j];
        if(t[i] == p[j+1]) j++;
        if(j == len_p - 1) return 1;
    }
    return 0;
}

char str[MAXN][MAX_LEN];

char ans[MAX_LEN];

char tmp[MAX_LEN];

int main()
{
    int n;
    while(~scanf("%d",&n) && n)
    {
        int min_len = 222;
        int min_str;
        for(int i = 0;i<n;i++)
        {
            scanf("%s",str[i]);
            int len = strlen(str[i]);
            if(len < min_len)
            {
                min_len = len;
                min_str = i;
            }
        }

        int l = 1,r = min_len;
        ans[0] = '\0';
        while(l <= r)
        {
            int mid = (l+r) >> 1;
            int ok = 0;
            int first = 1;
            for(int i = 0;i<min_len;i++)
            {
                int j = i + mid - 1;
                if(j >= min_len) break;
                int cnt = 0;
                for(int k = i;k<=j;k++)
                    tmp[cnt ++] = str[min_str][k];
                tmp[cnt] = '\0';
                int flag = 1;
                for(int k = 0;k<n;k++)
                    if(k != min_str)
                    {
                        if(!kmp(str[k],tmp))
                        {
                            flag = 0;
                            break;
                        }
                    }
                if(flag)
                {
                    ok = 1;
                    if(first || strcmp(ans,tmp) > 0)
                    {
                        strcpy(ans,tmp);
                        first = 0;
                    }
                }
            }
            //printf("l = %d,r = %d,mid = %d\n",l,r,mid);
            if(ok)
            {
                l = mid + 1;
            }
            else r = mid - 1;
        }
        if(ans[0] != '\0') puts(ans);
        else puts("IDENTITY LOST");
    }
    return 0;
}