題目

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]]

思路

dfs 的方式遞迴，有數字填入時，k - 1，當 k = 0 時結束。

Solution

/**
 * @param {number} n
 * @param {number} k
 * @return {number[][]}
 */
var combine = function(n, k) {
    const res = []
    
    var dfs = function(n, k, arr) {
    
        if (k === 0) return res.push(arr)
        if (k > n) return

        dfs(n - 1, k - 1, [...arr, n])
        dfs(n - 1, k, arr)
    }
    
    dfs(n, k, [])
    
    return res
};

Complexity

• Time complexity : O(n * k)
• Space complexity : O(n * k)