Combinations

Description

Given two integers n and k, return all possible combinations of k numbers chosen from the range [1, n].

You may return the answer in any order.

Example 1:

Input: n = 4, k = 2 Output: [[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]] Explanation: There are 4 choose 2 = 6 total combinations. Note that combinations are unordered, i.e., [1,2] and [2,1] are considered to be the same combination.

Example 2:

Input: n = 1, k = 1 Output: 1 Explanation: There is 1 choose 1 = 1 total combination.

Constraints:

• 1 <= n <= 20
• 1 <= k <= n

Code

介於 Subsets 和 Permutations 之間（以所求的 set size 的而言）。

class Solution {
public:
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> answer;
        vector<int> curr;
        vector<int> nums;
        for(int i = 1; i <= n; i++) {
            nums.push_back(i);
        }
        helper(answer, nums, 0, curr, k);
        return answer;
    }
 
    void helper(vector<vector<int>> &answer, vector<int> nums, int index, vector<int> curr, int k) {
        
        if(curr.size() == k) {
            answer.push_back(curr);
            return;
        }
            
        for(int i = index; i < nums.size(); i++) {
            curr.push_back(nums[i]);
            // backtracking
            helper(answer, nums, i + 1, curr, k);
            curr.pop_back();
        }
        
    }
};

Source

• Combinations - LeetCode