1313 - Decompress Run-Length Encoded List.md

Problem

1313 - Decompress Run-Length Encoded List

Problem Description

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]

My solution

In order to solve this problem, my approach is to first calculate the length of the final array.

Once I have that, I create the array that I will return as the final answer later. Now, using a for loop, I iterate through all the even numbers of the original array. Inside this loop, we have a nested for loop which goes from 0 to nums[i] and adds nums[i + 1] to the res array that I created before. Finally I return res.

Another possible approach is to not start by calculating the final length and instead use a List to store all the values and, at the end, convert the List to a static array and then return it, though I tried this implementation and was less efficient in time.

Code

• Language: Java

class Solution {
    public int[] decompressRLElist(int[] nums) {
        int len = 0, k = 0;

        for (int i = 0; i < nums.length; i += 2)
            len += nums[i];


        int[] res = new int[len];

        for (int i = 0; i < nums.length; i += 2)
            for (int j = 0; j < nums[i]; j++)
                res[k++] = nums[i + 1];

        return res;
    }
}

Complexity Analysis

• Time Complexity: O(N*M)
• Space Complexity: O(N)