leetcode[77] Combinations

给定n和k，从1到n中选k个数，存到结果中。其实就是组合问题。例如

If n = 3， k = 2, 结果是 { 1,2], [1,3], [2,3] };

思路：利用回溯法。

class Solution {
public:
    void dfs77(vector<vector<int > > &ans, vector<int> subans, int start, int n, int k)
    {
        if (subans.size() == k)
        {
            ans.push_back(subans); return ;
        }
        for (int i = start; i <= n; i++)
        {
            subans.push_back(i);
            dfs77(ans, subans, i + 1, n, k);
            subans.pop_back(); // 满足一个条件或者该分支递归完后要删除最后一个
        }
    }
    vector<vector<int> > combine(int n, int k) {
        vector<vector<int > > ans;
        if (n < k || k == 0) return ans;
        vector<int> subans;
        dfs77(ans, subans, 1, n, k);
        return ans;
    }
};