【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 + 1, 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() ? 1 : path.back() + 1; i <= n - k + 1 + 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。