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;

    }

};