Given a list pairs of [value, count] pairs, rebuild the original list: each pair contributes count copies of value, in order. A count of 0 contributes nothing.
Examples
Input: pairs = [[1, 2], [2, 1], [3, 3]]
Output: [1, 1, 2, 3, 3, 3]
Input: pairs = [[9, 0], [4, 2]]
Output: [4, 4]
Constraints
0 <= len(pairs) <= 10**5, all counts>= 0, total output length<= 10**5- Return a new list.
- Target: O(output length) time.
Goals
- Expand each pair into repeated values with extend
- Handle a zero count that contributes nothing
- Build the result in a single pass