Problem

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.

Examples

Example 1:

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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:

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

Solution

Method 1 - Simple iteration - Two Pass

Code

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class Solution {
    public int[] decompressRLElist(int[] nums) {
        int arrSize = 0;
        for (int i = 0; i < nums.length; i += 2) {
            arrSize += nums[i];
        }
        
        int[] result = new int[arrSize];

        int startIdx = 0;
        for (int i = 0; i < nums.length; i += 2) {
            Arrays.fill(result, startIdx, startIdx + nums[i], nums[i + 1]);
            startIdx += nums[i];
        }
        return result;
    }
}

Complexity

  • ⏰ Time complexity: O(n)
  • 🧺 Space complexity: O(1)