77. Combinations (opens new window)

Description

Difficulty: Medium

Related Topics: Backtracking (opens new window)

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

Language: JavaScript

/**
 * @param {number} n
 * @param {number} k
 * @return {number[][]}
 */
var combine = function(n, k) {
    const ans = [];
    
    const backtrack = (start = 1, path = []) => {
        if (path.length === k) return ans.push([...path]);
        
        for (let i = start; i <= n; i++) {
            backtrack(i + 1, [...path, i]);
        }
    }
    
    backtrack();
    
    return ans;
};