[?]*Combination(递归调用好难)

题目：

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],

]

 题解：

 这道题就是用DFS(参考Work BreakII)的循环递归处理子问题的方法解决。n为循环的次数，k为每次尝试的不同数字。用到了回溯。

 代码如下： 

 1 public ArrayList<ArrayList<Integer>> combine(int n, int k) {

 2         ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();

 3         if(n <= 0||n < k)

 4             return res;

 5         ArrayList<Integer> item = new ArrayList<Integer>();    

 6         dfs(n,k,1,item, res);//because it need to begin from 1

 7         return res;

 8     }

 9     private void dfs(int n, int k, int start, ArrayList<Integer> item, ArrayList<ArrayList<Integer>> res){

10         if(item.size()==k){

11             res.add(new ArrayList<Integer>(item));//because item is ArrayList<T> so it will not disappear from stack to stack

12             return;

13         }

14         for(int i=start;i<=n;i++){

15             item.add(i);

16             dfs(n,k,i+1,item,res);

17             item.remove(item.size()-1);

18         }

19     }

Reference:http://blog.csdn.net/linhuanmars/article/details/21260217

 http://www.cnblogs.com/springfor/p/3877168.html