Browser History with Doubly Linked List

Implement a browser history navigator backed by a doubly linked list.

Each node holds a URL and two pointers (prev, next). The cursor always
points at the current page. Visiting a new URL appends a node after the
cursor and discards everything forward (just like a real browser).

API

class BrowserHistory {
  constructor(homepage)     // start on this URL
  visit(url)                // move forward; drop all forward history
  back(steps)               // move backward up to `steps` pages; return current URL
  forward(steps)            // move forward up to `steps` pages; return current URL
}

solve(homepage, ops) replays operations and returns the current URL after
every back and forward call (visit produces null in the output).

Example

solve('home.com', [
  ['visit',   'a.com'],
  ['visit',   'b.com'],
  ['back',    1],         // → 'a.com'
  ['back',    1],         // → 'home.com'
  ['forward', 1],         // → 'a.com'
  ['visit',   'c.com'],   // → null  (b.com is now gone)
  ['forward', 100],       // → 'c.com'  (already at end)
  ['back',    2],         // → 'home.com'
])
// output: [null, null, 'a.com', 'home.com', 'a.com', null, 'c.com', 'home.com']

Constraints

• Steps may exceed the available history in either direction — clamp at the

boundary.

• The internal DListNode must carry both prev and next pointers.