[?]*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： 

public ArrayList<ArrayList<Integer>> combine(int n, int k) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
        if(n <= 0||n < k)
            return res;
        ArrayList<Integer> item = new ArrayList<Integer>();    
        dfs(n,k,1,item, res);//because it need to begin from 1
        return res;
    }
    private void dfs(int n, int k, int start, ArrayList<Integer> item, ArrayList<ArrayList<Integer>> res){
        if(item.size()==k){
            res.add(new ArrayList<Integer>(item));//because item is ArrayList<T> so it will not disappear from stack to stack
            return;
        }
        for(int i=start;i<=n;i++){
            item.add(i);
            dfs(n,k,i+1,item,res);
            item.remove(item.size()-1);
        }
    }

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

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