Given a board of `NxN`, distributed with tiles labeled `0` to `N² - 1`(inclusive):
A solved grid will have the tiles in order of label, left to right, top to bottom.
Return `true` if the board state is currently solved, and `false` if the board state is unsolved.
Input will always be a square 2d array.
For example, a 2x2 solved grid:
```
[ [0, 1],
  [2, 3] ]
```
A 2x2 unsolved grid:
```
[ [2, 1],
  [0, 3] ]
```
def is_solved(board):
    board = sum(board, [])
    return board == sorted(board)
def is_solved(board):
    board = sum(board, [])
    return board == sorted(board)