77. Combinations

递归+回溯 挺简单的吧

class Solution {
public:
  vector<vector<int>>res;
  vector<int>num;
  vector<int>temp;
  void combination(int index, int k) {
    if (k == 0) {
      res.push_back(temp);
      return;
    }
    for (int i = index; i < num.size(); ++i) {
      temp.push_back(num[i]);
      combination(i + 1,k - 1);
      temp.pop_back();
    }
  }
  vector<vector<int>> combine(int n, int k) {
    for (int i = 1; i <= n; ++i)num.push_back(i);
    combination(0, k);
        return res;
  }
};