Combinations

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

For example,
If n = 4 and k = 2, a solution is:

[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

又是一个很简单的题目啊！！递归。注意边界？

class Solution {
public:

    vector<vector<int> > r;
    int maxk;
    int maxn;
    
    void dfs(int left,int d,vector<int> &topVec)
    {
        if(d==maxk && left<=maxn)
        {
            topVec.push_back(left);
            r.push_back(topVec);
            topVec.pop_back();
            return;
        }
        else if(maxn-left<maxk-d) return;
        
        topVec.push_back(left);
        
        for(int li=left+1;li<=maxn-maxk+d+1;li++)
        {
            dfs(li,d+1,topVec);
        }
        topVec.pop_back();
    }
    vector<vector<int> > combine(int n, int k) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        r.clear();
        vector<int> topVec;
        
        maxk=k;
        maxn=n;
        
        for(int li=1;li<=n;li++)
        {
            dfs(li,1,topVec);
        }
        
        return r;
    }
};