组给出两个整数n和k，返回从1……n中选出的k个数的组合。

样例

例如 n = 4 且 k = 2

返回的解为：

[[2,4],[3,4],[2,3],[1,2],[1,3],[1,4]]

分析：对每个元素，取或不取，递归回溯

代码：

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> > combine(int n, int k) {
        // write your code here
        vector<vector<int> > ret;
        vector<int> cur;
        combine(cur,1,n,k,ret);
        return ret;
    }
    void combine(vector<int> cur,int index,int n,int k,vector<vector<int> >&ret)
    {
        if(cur.size()==k)
        {
            ret.push_back(cur);
            return;
        }
        if(index>n||cur.size()>k)
            return;
        combine(cur,index+1,n,k,ret);
        cur.push_back(index);
        combine(cur,index+1,n,k,ret);
    }
    
};