77. 组合

 

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class Solution {
    void help(int index,int n,int k,vector&tmp,vector>&res)
    {
        if(tmp.size()==k)
        {
            res.push_back(tmp);
            return;
        }
        //树的第一层
        for(int i=index;i<=n;i++)
        {
            
            tmp.push_back(i);
            //树的第二层
            help(i+1,n,k,tmp,res);
            tmp.pop_back();
            


        }
    }
public:
    vector> combine(int n, int k) {
        
        vector>res;
        if(n==0||k==0)return res;
        vectortmp;
        help(1,n,k,tmp,res);

        return res;
    }
};