codeforces 126B Password

题意：给你一个字符串 ，问你既是它的前缀 ，又是它的后缀，且是在中间出线过的最长字串是什么

解题思路：KMP变形，不熟悉next写出来还是有点困难

解题代码：

 1 // File Name: 126b.cpp
 2 // Author: darkdream
 3 // Created Time: 2015年03月07日 星期六 15时23分51秒
 4
 5 #include<vector>
 6 #include<list>
 7 #include<map>
 8 #include<set>
 9 #include<deque>
10 #include<stack>
11 #include<bitset>
12 #include<algorithm>
13 #include<functional>
14 #include<numeric>
15 #include<utility>
16 #include<sstream>
17 #include<iostream>
18 #include<iomanip>
19 #include<cstdio>
20 #include<cmath>
21 #include<cstdlib>
22 #include<cstring>
23 #include<ctime>
24 #define LL long long
25
26 using namespace std;
27 char str[1000005];
28 int dp[1000005];
29 int hs[1000005];
30 void getnext(char *p,int next[])
31 {
32    int pLen =strlen(p);
33    next[0] = -1;
34    int k = -1;
35    int j = 1;
36    while(j < pLen )
37    {
38      if(k == -1 || p[j] == p[k+1])
39      {
40       if(p[j] == p[k+1])
41       {
42          next[j] = k + 1;
43          k ++;
44          j ++ ;
45       }else{
46          next[j] = -1;
47          j++;
48       }
49      }
50      else {
51         k = next[k] ;
52      }
53    }
54 }
55 int main(){
56    scanf("%s",str);
57    getnext(str,dp);
58    int len = strlen(str);
59    for(    int i = 0;i < len; i++)
60    {
61      /*if(dp[i] == 0 && str[i] == str[0])
62      {
63
64      }
65      else
66        dp[i]--;
67      *///printf("%d ",dp[i]);
68        if(dp[i] != -1)
69        hs[dp[i]] ++;
70    }
71    if(dp[len-1] != -1)
72       hs[dp[len-1]] -- ;
73    while(hs[dp[len-1]] ==0 && dp[len-1] != -1)
74    {
75   //     printf("dp[len-1] = %d\n",dp[len-1]);
76        dp[len-1] = dp[dp[len-1]];
77    }
78   // printf("ans = %d\n",dp[len-1]);
79    int an = 0 ;
80    for(int i = len -2;i >= 1;i --)
81        if(dp[i] == dp[len -1])
82        {
83            an = 1 ;
84        }
85    if(an == 0 || dp[len -1] == -1 || (dp[len-1] ==0 && str[len-1] != str[0]))
86    {
87        printf("Just a legend\n");
88        return 0 ;
89    }
90    for(int i =0 ;i <= dp[len-1];i ++)
91        printf("%c",str[i]);
92
93 return 0;
94 }