combination

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 {
private:
    vector<vector<int> > ret;
    vector<int> a;
public:
    void solve(int dep, int maxDep, int n, int start)
    {
        if (dep == maxDep)
        {
            ret.push_back(a);
            return;
        }
        
        for(int i = start; i <= n; i++)
        {
            a[dep] = i;
            solve(dep + 1, maxDep, n, i + 1);
        }      
    }
    
    vector<vector<int> > combine(int n, int k) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        a.resize(k);
        ret.clear();
        solve(0, k, n, 1);
        return ret;
    }
};
void getCombine(  
                int n,  
                int k,  
                int level,  
                vector<int> &solution,  
                vector<vector<int>> &result){  
    if(solution.size() == k){  
        result.push_back(solution);  
        return;  
    }  
    for(int i = level; i<=n; i++){  
        solution.push_back(i);  
        getCombine(n,k,i+1,solution,result);  
        solution.pop_back();  
    }  
}  
vector<vector<int> > combine(int n, int k) {  
    vector<int> solution;  
    vector<vector<int>> result;  
    getCombine(n,k,1,solution,result);  
    return result;  
}