[LeetCode] Combinations

题目描述:

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],
]

解题思路:

class Solution {
public:
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> result;
        vector<int> elem;
        
        combine(n, k, 1, result, elem);
        return result;
    }
private:
    void combine(int n, int k, int cur, vector<vector<int>> &result, vector<int> &elem) {
        if (elem.size() == k) {
            result.push_back(elem);
            return;
        }
        
        for (int i = cur; i <= n; ++i) {
            elem.push_back(i);
            combine(n, k, i + 1, result, elem);
            elem.pop_back();
        }
    }
};