Strings — Simplify Path

solve(path) — path is an absolute Unix-style path starting with /. Return its
canonical form:

• one slash between directory names, no trailing slash (except for the root "/");
• . means the current directory and is dropped;
• .. goes up one level (and does nothing at the root);
• several consecutive slashes count as one;
• any other name, including `...`, is a normal directory name.

Split on /, then process the pieces with a stack: ignore empty pieces and ., pop on
.. (if the stack is not empty), push everything else. Join the stack back with /.

Examples

solve("/home/")                → "/home"
solve("/home//foo/")           → "/home/foo"
solve("/a/./b/../../c/")       → "/c"
solve("/../")                  → "/"
solve("/...")                  → "/..."