Grep with context lines

Implement a simplified grep. You get a list of text lines, a search string target, and a non-negative integer k. Return every line that contains target, together with up to k lines before it and up to k lines after it.

def grep_with_context(lines: list[str], target: str, k: int) -> list[str]: ...

Rules:

• A line matches if target appears anywhere in it as a case-sensitive substring, including inside a longer word. "log" matches "catalog" but not "LOG".
• Context windows stop at the start and end of the input.
• Each line appears at most once in the result, even if it is in the context of several matches.
• The result keeps the original line order.

Constraints: 0 <= len(lines) <= 10^5, each line up to 10^3 characters, 0 <= k <= 10^5.

Example 1

lines  = ["boot ok", "disk warn", "net ok", "cpu ok", "fan warn", "done"]
target = "warn", k = 1
output = ["boot ok", "disk warn", "net ok", "cpu ok", "fan warn", "done"]

The windows are lines 0-2 and 3-5. Together they cover every line, each once.

Example 2

lines  = ["a", "bx", "c", "d", "e", "fx", "g"]
target = "x", k = 0
output = ["bx", "fx"]

Be ready for follow-ups: different before/after counts, a -- separator between groups, and input too large to hold in memory.

Reconstructed from the reported question; details may differ from what interviewers ask.