Resolve variable placeholders
A CI pipeline defines variables as lines of the exact form "KEY=value" (no comments, keys need no trimming; split on the first "="). A value may embed placeholders written as "${NAME}". Resolve every placeholder exactly once: substitute the RAW value of that key as written in the file — do not re-resolve anything inside the substituted text. A placeholder naming a key that does not exist becomes the empty string. A "${" with no closing "}" is kept verbatim through the end of the value, and a "$" not followed by "{" is ordinary text. Return the dict of resolved values.
resolve_placeholders(lines: list[str]) → dict[str,str][["HOST=db.local","URL=postgres://${HOST}:5432"]]out{"URL":"postgres://db.local:5432","HOST":"db.local"}State your approach and its time/space complexity out loud before you optimize. Handle the edge cases (empty input, duplicates, overflow), and say why you chose this over the brute force. Green tests are the floor, not the grade.
[["HOST=db.local","URL=postgres://${HOST}:5432"]]{"URL":"postgres://db.local:5432","HOST":"db.local"}not run yetsample