Leetcode•Feb 07, 2026
Decompress Run-Length Encoded List
Hazrat Ali
Leetcode
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]
Solution
var decompressRLElist = function(nums) {
let result = [];
for (let i = 0; i < nums.length; i += 2) {
let freq = nums[i];
let val = nums[i + 1];
for (let j = 0; j < freq; j++) {
result.push(val);
}
}
return result;
};