The Way from Here to There

In a file system, every file has an absolute path such as /home/ana/notes/todo.txt: its parts are separated by /, and / alone is the root. A relative path says how to get somewhere from a given directory instead: .. means "go up one directory" and a name means "go down into it".

Write relative_path(start, target) that returns the shortest relative path from directory start to target, both absolute:

• First normalise both paths: empty parts (from // or a trailing /) and . parts are ignored, and .. removes the part before it (at the root, .. stays at the root).
• The result is made of .. parts, then names, joined with /, and it never ends with /.
• If start and target are the same directory, return ".".

Examples

Input:  start = "/home/ana/projects/site", target = "/home/ana/notes/todo.txt"
Output: "../../notes/todo.txt"

Input:  start = "/", target = "/usr/local/bin"
Output: "usr/local/bin"

Input:  start = "/data/raw/", target = "/data/./raw//2024/../2025"
Output: "2025"
Explanation: The target normalises to /data/raw/2025.

Input:  start = "/a/bc", target = "/a/b"
Output: "../b"
Explanation: "bc" and "b" are different directories, even though one name starts with the other.

Constraints

• Both paths start with / and have at most 10**4 parts.
• Names contain no /; a part is only special when it is exactly . or ...

Goals

• Normalise a path by splitting it into parts and resolving `.` and `..`
• Find the length of a common prefix of two lists by walking them together with `zip`
• Build the answer from repeated `..` parts and a slice, then join once