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],
]

代碼例如以下：

public class Solution {
    boolean[] f;
    List<List<Integer>> list;
    public List<List<Integer>> combine(int n, int k) {
        list = new ArrayList<List<Integer>>();
        
        if(k > n || k < 1){//必須是有效k值
            return list;
        }
        f = new boolean[n];
        int[] a = new int[n];
        for(int i=0; i < n; i++){
            a[i] = i+1;//將數填充
        }
        mm = n-1;
        count(a,"",k,n,0);//調用函數
        return list;
    }
    /**
     * 實現對k個數字的排練組合
     * @param a 數組
     * @param s 排練組合得到的結果
     * @param nn 排練組合的數字個數
     */
    int kk = 0;
    int mm;
    private void count(int[] a,String s,int nn,int n,int j){
        if(nn == 0){//處理結果
            String[] sa = s.split(",");//切割數組
            List<Integer> al = new ArrayList<Integer>();
            for(int i = 0;i < sa.length; i++){
                if(sa[i].length() > 0)//僅僅有當不為空的時候才加入
                	al.add(Integer.parseInt(sa[i]));//加入
            }
            list.add(al);
            return;
        }
        //遍歷,從i=j開始，僅僅要i開頭的與i大的值
        for(int i = j; i < a.length; i++){
            if(!f[i]){
                f[i] = true;
                count(a,s + "," + a[i],nn-1,n,i+1);//調用下一個為false的數字
                f[i] = false;
            }
        }
    }
}

leetCode 77.Combinations (組合)