77. Combinations

class Solution 
{
public:
    vector<vector<int>> combine(int n, int k) 
    {
        vector<vector<int>> res;
        int i=0;
        vector<int> cur(k,0);
        while(i>=0)
        {
            cur[i]++;
            if(cur[i]>n)
                i--;
            else if(i==k-1)
                res.push_back(cur);
            else
            {
                i++;
                cur[i]=cur[i-1];
            }
        }
        return res;
    }
};

从前向后推，以升序序列为基准构成答案序列