组合

给定两个整数 n 和 k. 返回从 1, 2, ... , n 中选出 k 个数的所有可能的组合.

样例

样例 1:

输入: n = 4, k = 2
输出: [[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]]

样例 2:

输入: n = 4, k = 1
输出: [[1],[2],[3],[4]]

注意事项

你可以以任意顺序返回所有的组合, 但是一个组合内的所有数字需要是升序排列的.

 

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> tmp;
        vector<int> nums;
        for(int i = 1;  i <=n; i++)
        {
            nums.push_back(i);
        }
        //sort(nums.begin(), nums.end());
        backtrack(ret, tmp, nums, 0,k);
        return ret;
    }
    
    void backtrack(vector<vector<int>>& ret, vector<int> &tempList, vector<int>& nums, int start,int k)
    {
        if(tempList.size() == k)
            ret.push_back(tempList);
        for(int i = start; i < nums.size(); i++)
        {
            tempList.push_back(nums[i]);
            backtrack(ret, tempList, nums, i+1,k);
            tempList.erase(tempList.begin() + tempList.size() - 1);
            
        }
    }
};