This page is still under construction.

Parts of this page are still being built. What you see may change.

Tic-tac-toe

Interview

Time limit5sMemory limit512 MB

Summary
For each 3x3 tic-tac-toe board, decide whether it is invalid, reachable only by non-optimal play, or reachable by two perfect players.
Level

Medium7 of 10

Topics
Game theory, Simulation, Implementation, Brute force
Solved
No attempts yet

Problem

A group of archaeologists has found an ancient clay tablet with a game of Tic-tac-toe engraved on it.

Tic-tac-toe is a paper-and-pencil game for two players, 'O' and 'X', who take turns marking the spaces in a 3×33 \times 3 grid. The player who succeeds in placing three respective marks in a horizontal, vertical, or diagonal row wins the game.

You, as an employee of the State Historical Museum in Byteozavodsk, are to arbitrate if this state of game could have been created by two excellent players.

Input

The first line of input contains a single positive integer tt, the number of test cases. The descriptions of the test cases follow.

Each test case consists of three lines, three characters on each line. The jj-th character of the ii-th line denotes the state of the jj-th square in the ii-th row of the clay tablet. There are three possibilities:

  • "." denotes an empty square,
  • "O" (big "o") denotes a square that was marked by first player,
  • "X" denotes a square that was marked by second player.

Each test case is preceded by a single blank line.

Output

For each test case, output a single line containing a single word: "INVALID", if there does not exist a valid sequence of alternating moves that leads to this game state, "UNREACHABLE", if there does exist a valid sequence of alternating moves that leads to this game state, but only when at least one of players is not excellent, and "REACHABLE" otherwise.

Hint

Provided that it is possible, an excellent player always performs a move that lets him win regardless of further moves of his opponent. If it is not possible, then he performs a move that brings him to a draw. If worse comes to worst, he makes any move.

Examples1

  1. Example 1

    Input
    3
    
    ...
    .X.
    ...
    
    ...
    .OX
    ...
    
    ...
    .O.
    ..X
    
    Expected output
    INVALID
    UNREACHABLE
    REACHABLE