【LeetCode】combinations

版权声明：噗噗个噗~~~ https://blog.csdn.net/Puyar_/article/details/85320272

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],
]

class Solution {
public:
    vector<vector<int> > combine(int n, int k) {
       vector<vector<int>>res;
        vector<int>ans;
        build(1,n,k,ans,res);
        return res;
    }
    void build(int num,int n,int k,vector<int>&ans,vector<vector<int>>&res)
    {
        if(k==0){
            res.push_back(ans);
            return;
        }
        if(num>n)return;
        ans.push_back(num);
        build(num+1,n,k-1,ans,res);
        ans.pop_back();
        build(num+1,n,k,ans,res);
    }   
};