Combinations

Q：

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],
]

A：

费了九虎二牛之力写了这么个diao程序，烦躁了。

class Solution {
 public:
  vector<vector<int> > combine(int n, int k) {
    // Start typing your C/C++ solution below
    // DO NOT write int main() function
    if (k > n) {
      return vector<vector<int> >();
    } 
    return combineInternal(1, n, k);
  }
 private:
  vector<vector<int> > combineInternal(int begin, int end, int k) {
    vector<vector<int> > results;
    if (end - begin < k - 1) {
      return results;
    }   
    if (end - begin == k - 1) {
      vector<int> result;
      for (int i = begin; i <= end; ++i) {
        result.push_back(i);
      }   
      results.push_back(result);
      return results;
    }   
    if (k == 1) {
      vector<int> result;
      result.resize(1);
      for (int i = begin; i <= end; ++i) {
        result[0] = i;
        results.push_back(result);
      } 
      return results;
    } 
    for (int i = begin; i <= end - k + 1; ++i) {
      vector<int> result;
      result.push_back(i);
      vector<vector<int> > part_results = combineInternal(i + 1, end, k - 1);
      if (part_results.empty()) results.push_back(result);
      for (int j = 0; j < part_results.size(); ++j) {
        result.resize(1);
        result.insert(result.end(), part_results[j].begin(), part_results[j].end());
        results.push_back(result);
      }   
    }   
    return results;
  }
};