【Leetcode】Combinations

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>> result;
         vector<bool> used(n + , false);
         vector<int> path;
         dfs(n, k, used, path, result);
         return result;
     }
 private:
     void dfs(int n, int k, vector<bool> & used, vector<int> &path, vector<vector<int>> &result) {
         if (path.size() == k) {
             result.push_back(path);
             return;
         }
         for (int i = path.empty() ?  : path.back() + ; i <= n - k +  + path.size(); ++i) {
             if (!used[i]) {
                 used[i] = true;
                 path.push_back(i);
                 dfs(n, k, used, path, result);
                 path.pop_back();
                 used[i] = false;
             }
         }
     }
 };

与排列类似，只是dfs的时候的搜索的策略稍有不同。