77. 组合

给定两个整数 n 和 k，返回 1 ... n 中所有可能的 k 个数的组合。

示例:

输入: n = 4, k = 2
输出:
[
[2,4],
[3,4],
[2,3],
[1,2],
[1,3],
[1,4],
]

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/combinations
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

public class Solution {
    private boolean[] flag = null;
    private List<List<Integer>> res = null;

    private void helper(int n, int k, int cur,List<Integer> subset){
        if (k == 0) {
            res.add(new ArrayList<>(subset));
            return;
        }
        for (int i = cur; i <= n; i++) {
            if (!flag[i]){
                if (subset.size() > 0 && subset.get(subset.size()-1) >= i)
                    break;
                flag[i] = true;
                subset.add(i);
                helper(n, k-1, i+1, subset);
                flag[i] = false;
                subset.remove(subset.size()-1);
            }
        }
    }


    public List<List<Integer>> combine(int n, int k) {
        flag = new boolean[n+1];
        res = new ArrayList<>();
        List<Integer> subset = new ArrayList<>();

        helper(n,k,1,subset);
        return res;
    }

    public static void main(String[] args) {
        List<List<Integer>> combine = new Solution().combine(4, 2);
        for (List<Integer> e : combine) {
            System.out.println(e);
        }
    }
}