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;  
}