How should I practice Run-Length Decoding?

Start by reading the full statement, solve the visible examples, then compare the pattern against related Apple phone screen problems in FastPrep.

Is Run-Length Decoding associated with Apple?

FastPrep tags this as a Apple phone screen coding practice problem when the problem catalog has that company and stage context.

Run-Length Decoding

Apple● EasyFULLTIMEPHONE SCREEN

Learn

Problem statement

Decode a run-length encoded string encoded. Each token is one lowercase English letter followed by an optional positive decimal count. A missing count means 1. Emit the letter that many times and concatenate the runs in order.

For example, a5b3c expands to aaaaabbbc. Counts belong to the letter immediately before them and can have multiple digits. The empty string decodes to the empty string.

Function

decodeRuns(encoded: String) → String

Examples

Example 1

encoded = "a5b3c"return = "aaaaabbbc"

The original example uses postfix counts and an omitted count of one.

Example 2

encoded = ""return = ""

Empty encoded input expands to an empty string.

Example 3

encoded = "x12yz2"return = "xxxxxxxxxxxxyzz"

Multi-digit counts and consecutive omitted counts.

Constraints

The input is valid and contains only lowercase English letters and decimal digits.
Every count is positive, has no leading zero, and is at most 4096.
0 <= encoded.length <= 4096.
The decoded output has at most 4096 characters.