Leetcode #794: Valid Tic-Tac-Toe State
In this guide, we solve Leetcode #794 Valid Tic-Tac-Toe State in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
Given a Tic-Tac-Toe board as a string array board, return true if and only if it is possible to reach this board position during the course of a valid tic-tac-toe game. The board is a 3 x 3 array that consists of characters ' ', 'X', and 'O'.
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: Array, Matrix
Intuition
Grid problems are easiest when you define clear row/column boundaries.
A consistent traversal order prevents off-by-one errors.
Approach
Iterate by rows, columns, or layers depending on the requirement.
Keep bounds updated as the traversal progresses.
Steps:
- Define bounds or directions.
- Visit cells in order.
- Update result and move bounds.
Example
Input: board = ["O "," "," "]
Output: false
Explanation: The first player always plays "X".
Python Solution
class Solution:
def validTicTacToe(self, board: List[str]) -> bool:
def win(x):
for i in range(3):
if all(board[i][j] == x for j in range(3)):
return True
if all(board[j][i] == x for j in range(3)):
return True
if all(board[i][i] == x for i in range(3)):
return True
return all(board[i][2 - i] == x for i in range(3))
x = sum(board[i][j] == 'X' for i in range(3) for j in range(3))
o = sum(board[i][j] == 'O' for i in range(3) for j in range(3))
if x != o and x - 1 != o:
return False
if win('X') and x - 1 != o:
return False
return not (win('O') and x != o)
Complexity
The time complexity is O(m·n). The space complexity is O(1) to O(m·n).
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.