Combinations LeetCode Solution

Problem – Combinations

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

Combinations LeetCode Solution in C++

class Solution {
public:
	vector<vector<int>> combine(int n, int k) {
		vector<vector<int>> result;
		int i = 0;
		vector<int> p(k, 0);
		while (i >= 0) {
			p[i]++;
			if (p[i] > n) --i;
			else if (i == k - 1) result.push_back(p);
			else {
			    ++i;
			    p[i] = p[i - 1];
			}
		}
		return result;
	}
};

Combinations LeetCode Solution in Java

    public static List<List<Integer>> combine(int n, int k) {
		List<List<Integer>> combs = new ArrayList<List<Integer>>();
		combine(combs, new ArrayList<Integer>(), 1, n, k);
		return combs;
	}
	public static void combine(List<List<Integer>> combs, List<Integer> comb, int start, int n, int k) {
		if(k==0) {
			combs.add(new ArrayList<Integer>(comb));
			return;
		}
		for(int i=start;i<=n;i++) {
			comb.add(i);
			combine(combs, comb, i+1, n, k-1);
			comb.remove(comb.size()-1);
		}
	}

Combinations LeetCode Solution in Python

from itertools import combinations

class Solution:
    def combine(self, n, k):
        return list(combinations(range(1, n+1), k))
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