B. Making Genome in Berland

题目连接：

http://www.codeforces.com/contest/638/problem/B

Description

Berland scientists face a very important task – given the parts of short DNA fragments, restore the dinosaur DNA! The genome of a berland dinosaur has noting in common with the genome that we’ve used to: it can have 26 distinct nucleotide types, a nucleotide of each type can occur at most once. If we assign distinct English letters to all nucleotides, then the genome of a Berland dinosaur will represent a non-empty string consisting of small English letters, such that each letter occurs in it at most once.

Scientists have n genome fragments that are represented as substrings (non-empty sequences of consecutive nucleotides) of the sought genome.

You face the following problem: help scientists restore the dinosaur genome. It is guaranteed that the input is not contradictory and at least one suitable line always exists. When the scientists found out that you are a strong programmer, they asked you in addition to choose the one with the minimum length. If there are multiple such strings, choose any string.

Input

The first line of the input contains a positive integer n (1 ≤ n ≤ 100) — the number of genome fragments.

Each of the next lines contains one descriptions of a fragment. Each fragment is a non-empty string consisting of distinct small letters of the English alphabet. It is not guaranteed that the given fragments are distinct. Fragments could arbitrarily overlap and one fragment could be a substring of another one.

It is guaranteed that there is such string of distinct letters that contains all the given fragments as substrings.

Output

In the single line of the output print the genome of the minimum length that contains all the given parts. All the nucleotides in the genome must be distinct. If there are multiple suitable strings, print the string of the minimum length. If there also are multiple suitable strings, you can print any of them.

Sample Input

3
bcd
ab
cdef

Sample Output

abcdef

Hint

题意

有一个dna片段，这个片段只含有26个英文字母，且这些字母最多出现一次

现在我们截取了n个片段，现在想让你还原出原来的dna片段是啥

请还原出最短的那个，保证有解

题解：

当成图论题来做就好了。

由于保证是截取的n个片段，所以直接dfs维护一个拓扑序列就好了

然后就完了……

代码

#include<bits/stdc++.h>
using namespace std;
const int maxn = 250;
vector<int> E[30];
int vis[maxn];
string ans;
void dfs(int x)
{
    vis[x]=2;
    for(int i=0;i<E[x].size();i++)
    {
        if(vis[E[x][i]]==2)continue;
        dfs(E[x][i]);
    }
    ans+=(char)('a'+x);
}
int main()
{
    int n;scanf("%d",&n);
    for(int i=1;i<=n;i++)
    {
        string s;cin>>s;
        for(int j=0;j<s.size()-1;j++)
            E[s[j]-'a'].push_back(s[j+1]-'a'),vis[s[j+1]-'a']=3;
        if(vis[s[0]-'a']!=3)vis[s[0]-'a']=1;
    }
    for(int i=0;i<26;i++)if(vis[i]==1)dfs(i);
    reverse(ans.begin(),ans.end());
    cout<<ans<<endl;
}