LeetCode in Net

77. Combinations

Medium

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

You may return the answer in any order.

Example 1:

Input: n = 4, k = 2

Output: [ [2,4], [3,4], [2,3], [1,2], [1,3], [1,4], ]

Example 2:

Input: n = 1, k = 1

Output: [[1]]

Constraints:

• 1 <= n <= 20
• 1 <= k <= n

Solution

public class Solution {
    public IList<IList<int>> Combine(int n, int k) {
        var res = new List<IList<int>>();
        Backtrack(1, n, k, new List<int>(), res);
        return res;
    }
    private void Backtrack(int start, int n, int k, List<int> curr, List<IList<int>> res) {
        if (curr.Count == k) {
            res.Add(new List<int>(curr));
            return;
        }
        for (int i = start; i <= n; i++) {
            curr.Add(i);
            Backtrack(i + 1, n, k, curr, res);
            curr.RemoveAt(curr.Count - 1);
        }
    }
}

This site is open source. Improve this page.