Decompress run length encoded list

Time: O(N); Space: O(1); easy

We are given a list nums of integers representing a list compressed with run-length encoding.

Consider each adjacent pair of elements [freq, val] = [nums[2*i], nums[2*i+1]] (with i >= 0). For each such pair, there are freq elements with value val concatenated in a sublist. Concatenate all the sublists from left to right to generate the decompressed list.

Return the decompressed list.

Example 1:

Input: nums = [1,2,3,4]

Output: [2,4,4,4]

Explanation:

  • The first pair [1,2] means we have freq = 1 and val = 2 so we generate the array [2].

  • The second pair [3,4] means we have freq = 3 and val = 4 so we generate [4,4,4].

  • At the end the concatenation [2] + [4,4,4] is [2,4,4,4].

Example 2:

Input: nums = [1,1,2,3]

Output: [1,3,3]

Notes:

  • 2 <= len(nums) <= 100

  • len(nums) % 2 == 0

  • 1 <= nums[i] <= 100

Hint:

Decompress the given array by repeating nums[2*i+1] a number of times equal to nums[2*i].

[1]:

class Solution1(object):
    def decompressRLElist(self, nums):
        """
        :type nums: List[int]
        :rtype: List[int]
        """
        return [nums[i+1] for i in range(0, len(nums), 2) for _ in range(nums[i])]

[2]:

s = Solution1()
nums = [1,2,3,4]
assert s.decompressRLElist(nums) == [2,4,4,4]
nums = [1,1,2,3]
assert s.decompressRLElist(nums) == [1,3,3]