Given two integers n and k, return all possible combinations of k numbers out of 1 … n.

Example
For example,
If n = 4 and k = 2, a solution is:
[[2,4],[3,4],[2,3],[1,2],[1,3],[1,4]]

很明显就是一个DFS+回溯的问题；
跟求全排列、幂集都很像；
思路就是想象递归的搜索解决树，搜索解决数每一层的可能解。

class Solution {
public:
    /**
     * @param n: Given the range of numbers
     * @param k: Given the numbers of combinations
     * @return: All the combinations of k numbers out of 1..n
     */

    vector<vector<int> > res;

    void helper(int n,int k,int cur,vector<int> tmp){

        if(tmp.size()==k){
            res.push_back(tmp);
            return;
        }

        //当前层有可选的数从cur到n都有可能
        for(int i=cur;i<=n;i++){

            tmp.push_back(i);
            helper(n,k,i+1,tmp);//i+1
            tmp.pop_back();

        }

    }

    vector<vector<int> > combine(int n, int k) {
        // write your code here
        vector<int> tmp;

        helper(n,k,1,tmp);

        return res;
    }
};

参考：
​​​https://siddontang.gitbooks.io/leetcode-solution/content/backtracking/combination.html​​