Focused on Performance. ( Reducing duplicate operation )
First step, create a tree which visits all cells.

1. start with (0,0) and expand forming initial tree.
2. find an unvisited cell (orphan) that's neighbor to tree.
   connect the cell to the tree and expand from the cell.
3. repeat 2) till all cells are visited.
   and expand since this c
   Second step, make more doors open. [Skipped]

In some cases, the exploring will stop even when not all the cells are visited.
And why not call explore(0,0) in the beginning?

I attempted with my solution.
The result is

Time: 344ms Passed: 24 Failed: 0 Errors: 1

Test Results:
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
✔ Test Passed
 Same path found in different mazes

What does this "Same path found in different mazes" mean?

Most of all, it doesn't return 0. 0 can be answer too. Isn't it?