UVa 10776

/**
    * UVa-10776
    * Author    : Arif Ahmad
    * Date      : 24-08-14
    * Algo      : Backtracking + Pruning
    * Difficulty: Mideum
*/

#include <bits/stdc++.h>
using namespace std;

#define INF_MAX     2147483647
#define INF_MIN     -2147483648
#define INF         (1 << 30)
#define EPS         1e-9
#define PI          acos(-1.0)
#define N           2 + 30
#define MOD         10000000007
#define sz(x)       (int)(x).size()
#define all(x)      (x).begin(), (x).end()
#define pb          push_back
#define mp          make_pair
#define ms(x, a)    memset((x), (a), sizeof(x))
#define F           first
#define S           second
#define rep(i,a,b)  for(int i=(a); i<(b); i++)
#define repC(i,x)   for(size_t i=0; i<x.size(); i++)
#define repIT(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)

typedef long long       LL;
typedef pair<int,int>   pii;
typedef vector<int>     vi;
typedef vector<string>  vs;
typedef vector<char>    vc;
typedef vector<bool>    vb;
typedef map<string,int> msi;
typedef map<int,int>    mii;
typedef map<char,int>   mci;
typedef map<int,string> mis;

template<class T> T Abs(T x) {return x>0 ? x : -x;}
template<class T> T Max(T a, T b) {return a>b ? a : b;}
template<class T> T Min(T a, T b) {return a<b ? a : b;}
template<class T> T gcd(T a, T b) {return (b ? gcd(b,a%b) : a);}
bool isVowel(char ch){ch=tolower(ch);return(ch=='a' || ch=='e' || ch=='i' || ch=='o' || ch=='u');}

int r, n;
string s;
char res[N];

void backtrack(int i, int j)
{
    if(i == r) {
        res[r] = '\0';
        puts(res);
        return;
    }

    for(; j<n; j++) {
        res[i] = s[j];
        backtrack(i+1, j+1);
        while(s[j] == s[j+1]) j++; // pruning
    }
}


int main()
{
    #ifndef ONLINE_JUDGE
        freopen("in.txt", "r", stdin);
    #endif

    while(cin >> s >> r) {
        n = s.size();
        sort(s.begin(), s.end());

        // backtrack(i, j) -> i: no. of items taken. j: next position to take
        backtrack(0, 0);
    }

    return 0;
}