File: find-winner-on-a-tic-tac-toe-game/solution.py

Date: 2026-06-06

Time: 16:48

find-winner-on-a-tic-tac-toe-game/solution.py

Purpose

Solves LeetCode 1275: Find Winner on a Tic Tac Toe Game. Given a sequence of moves on a 3x3 board, determines the game state: whether player A or B won, whether it's a draw, or whether the game is still pending.

Key Components

validateBinaryTreeNodes(moves) — Misnamed function (likely a copy-paste artifact from another solution). Despite the name, it implements tic-tac-toe winner detection.

• Input: moves: list[list[int]] — ordered [row, col] pairs. Even-indexed moves belong to player A (value 1), odd-indexed to player B (value 2).
• Output: One of "A ", "B ", "Draw ", "Pending " (note trailing spaces — matches LeetCode's expected output format).

wins — A hardcoded list of all 8 winning lines: 3 rows, 3 columns, 2 diagonals. Each line is a list of (row, col) tuples.

grid — A 3x3 matrix initialized to zeros, representing the board. 0 = empty, 1 = A, 2 = B.

Patterns

• Eager win-check: After every move, all 8 winning lines are checked. This avoids tracking partial state but costs O(8×3) per move. Fine for a 3x3 board.
• Player alternation via index parity: player = 1 if i % 2 == 0 else 2 encodes the A-then-B turn order without separate state.
• Exhaustive line enumeration: Rather than computing rows/columns/diagonals dynamically, all 8 lines are pre-listed. Clean for a fixed-size board.

Dependencies

Imports: None — pure Python, no standard library usage.

Imported by: The test_solution.py in the same directory, plus — per the "Imported By" list — hundreds of unrelated test files. That massive import list is almost certainly a tooling artifact (e.g., a shared test harness or auto-generated import), not a real dependency on this solution's logic.

Flow

1. Initialize a 3x3 zero grid.

2. Iterate through moves with index i:

• Determine player from i % 2.
• Place the player's marker on the grid.
• Check all 8 winning lines: if any line is fully occupied by the current player, return that player's label immediately.

3. After all moves are played: return "Draw " if 9 moves were made (board full), otherwise "Pending ".

Invariants

• Moves are assumed valid: no out-of-bounds indices, no duplicate positions. The code does not validate input.
• Player A always moves first (index 0, 2, 4, ...).
• Win detection happens immediately after each move, so the first player to complete a line wins — later moves are never evaluated.
• The return strings have trailing spaces ("A ", not "A").

Error Handling

None. Invalid input (out-of-range coordinates, overlapping moves) will silently produce wrong results or raise an IndexError. This is typical for LeetCode solutions where input validity is guaranteed by the problem constraints.