Substring，

Problem

Dr lee cuts a string S into N pieces,s[1],…,s[N].   

Now, Dr lee gives you these N sub-strings: s[1],…s[N]. There might be several possibilities that the string S could be. For example, if Dr. lee gives you three sub-strings {“a”,“ab”,”ac”}, the string S could be “aabac”,”aacab”,”abaac”,…   

Your task is to output the lexicographically smallest S. 

Input

        The first line of the input is a positive integer T. T is the number of the test cases followed.   

The first line of each test case is a positive integer N (1 <=N<= 8 ) which represents the number of sub-strings. After that, N lines followed. The i-th line is the i-th sub-string s[i]. Assume that the length of each sub-string is positive and less than 100. 

Output

The output of each test is the lexicographically smallest S. No redundant spaces are needed. 

Sample input

1
3
a
ab
ac

Sample output

aabac

大意是给一组字符串, 把它们连接成一个字典次序最小的串, 一开始想法是按字典顺序sort这组字符串,也就是用STL中的string类中默认的比较(运算符i.e. >)函数,  可是这样的结果是wrong answer,  想出了一个(类)反例, 也可能是这种方法的唯一(类)的反例, 就是给你一组 'fu',  'fuck',  假如直接按字典顺序排序的话, 合成的字符串将是“fufuck”..
因为("fu"<"fuck")...而很明显("fuckfu"<"fufuck")..
所以需要改变那个比较函数, 定义一个string类的比较函数,
A < B  if ( A＋B < B ＋A)        //  ( A，B是字符串 ‘＋’是指字符串连接)
else A > B...

实现中用到STL中的sort, 跟string, 代码不用很长

代码如下,
//substring 
#include<vector>
#include<algorithm>
#include<iostream>
#include<string>

using namespace std;

bool str_cmp(string s1, string s2)
...{
    return (s1+s2<s2+s1);
}

int main()
...{
    int t, str_num, i, j; 
    cin>>t; 
    for(i=0; i<t; i++)
    ...{
        vector<string> str;
        vector<string>::iterator itr;
        cin>>str_num;
        string temp;
        for(j=0; j<str_num; j++)
        ...{
            cin>>temp;
            str.push_back(temp);            
        }
        sort(str.begin(), str.end(), str_cmp);
        for(itr=str.begin(); itr!=str.end(); itr++)
        ...{
            cout<<*itr;
        }
        cout<<endl;
    }
    return 0;
}


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