判定能否将一组单词排列在一个列表中，使得任何单词首字母与前面单词尾字母相同

#include <vector>
#include <cstring>

bool CanArrangeWords( size_t num, const char* arr[] )
{
    std::vector<size_t> s; // 其中的size_t为下标的两倍，以区分从前面插入还是从后面插入
    char head = arr[0][0];
    char tail = arr[0][strlen(arr[0])-1];
    size_t next = 2; // 为下标的两倍，以区分从前面插入还是从后面插入
    std::vector<bool> used_mark( num );

    while( s.size() < num-1 )
    {
        // 增长
        size_t t = s.size();
        for( size_t i=next; i!=num*2; ++i )
        {
            if( used_mark[i/2] )
                continue;

            if( i%2==0 && arr[i/2][strlen(arr[i/2])-1]==head )
            {
                s.push_back( i );
                head = arr[i/2][0];
                next = 2;
                used_mark[i/2] = true;
                break;
            }

            if( i%2==1 && arr[i/2][0]==tail )
            {
                s.push_back( i );
                tail = arr[i/2][strlen(arr[i/2])-1];
                next = 2;
                used_mark[i/2] = true;
                break;
            }
        }

        // 若无法增长则减短
        if( t == s.size() )
        {
            if( s.empty() )
                return false;

            size_t i = s.back();
            s.pop_back();
            if( i%2 == 0 )
                head = arr[i/2][strlen(arr[i/2])-1];
            else
                tail = arr[i/2][0];
            next = i + 1;
            used_mark[i/2] = false;
        }
    }

    return true;
}

#include <iostream>
using namespace std;

int main( void )
{
    //const char* s[] = { "fgg", "def", "abd", "gpjd" };
    const char* s[] = { "abc", "cba", "def", "gpjd" };

    cout << CanArrangeWords(_countof(s),s) << endl;

    return 0;
}