Combinations

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.
For example, If n = 4 and k = 2, a solution is:
[ [2,4], [3,4], [2,3], [1,2], [1,3], [1,4]]

题目大意：给定数字n和k，找出所有1->n中可能的k个数组合

题目难度：Medium

import java.util.*;

/**
 * Created by gzdaijie on 16/6/2
 */
public class Solution {
    private int n, k;
    private List<List<Integer>> result = new ArrayList<>();

    public List<List<Integer>> combine(int n, int k) {
        this.n = n;
        this.k = k;
        ArrayList<Integer> tmp = new ArrayList<>();
        combineRecursive(0, tmp);
        return result;
    }

    private void combineRecursive(int count, ArrayList<Integer> tmp) {
        if (count == k) {
            result.add((ArrayList<Integer>)tmp.clone());
            return;
        }

        int i = count > 0 ? tmp.get(count - 1) + 1 : 1;
        for (; i <= n; i++) {
            tmp.add(i);
            combineRecursive(count + 1, tmp);
            tmp.remove(count);
        }
    }
}