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Valid Sudoku Checker in Python

Aniruddha Chaudhari/2016/0
CodePython

Problem statement: You have given sudoku as matrix. For example,

sudoku validity checker example

If below three condition are valid then its a valid sudoku.

  • Each row will have items from 1 to 9.
  • Each column will have items from 1 to 9.
  • Each 3*3 cell (9 elements) will have items from 1 to 9.

Also, mention time and space complexity to validate sudoku.

This question was asked in the Protonn coding interview. This is one of the standard questions and asked in many coding challenges for software developer job profiles.

Python Solution

Here we are implementing sudoku matrix as list of list. Like,

sudoku = [[5,3,4,6,7,8,9,1,2],
               [6,7,2,1,9,5,3,4,8],
               [1,9,8,3,4,2,5,6,7],
               [8,5,9,7,6,1,4,2,3],
               [4,2,6,8,5,3,7,9,1], 
               [7,1,3,9,2,4,8,5,6],
               [9,6,1,5,3,7,2,8,4],
               [2,8,7,4,1,9,6,3,5],
               [3,4,5,2,8,6,1,7,8]]

Some of the tricks we have used to solve this problem.

  • We are using the built-in set() method to identify if all the elements in the list (sudoku row or sudoku column) are unique. Basically, the set() method removes all the duplicate elements from the list. If the length of the set is 9, it means there is no duplicate element.
  • We are using the list comprehension technique to get the column elements.
    [item[col_num] for item in sudoku]
  • A slicing mechanism from the Python list is used to get the elements for each cell.

Rest of the code is self-explanatory even if you know the Python basics.

#validate row
def isRowValid(row_num):
    return len(set(sudoku[row_num])) == 9

#validate column
def isColValid(col_num):
    col = [item[col_num] for item in sudoku]
    return len(set(col)) == 9

#validate cell
def isCelValid(cel_row, cel_col):
    vals = sudoku[cel_row][cel_col: cel_col+3]
    vals.extend(sudoku[cel_row+1] [cel_col: cel_col+3])
    vals.extend(sudoku[cel_row+2] [cel_col: cel_col+3])
    return len(set(vals)) == 9

#validate sudoku
def validateSudoku():
	for i in range(0,9):
		if not isRowValid(i):
			return False
		if not isColValid(i):
			return False
	for i in range(0, 9, 3):
		for j in range(0, 9, 3):
		    print(i, j)
		    if not isCelValid(i, j):
		        return False
	return True

sudoku = [[5,3,4,6,7,8,9,1,2],
              [6,7,2,1,9,5,3,4,8],
              [1,9,8,3,4,2,5,6,7],
              [8,5,9,7,6,1,4,2,3],
              [4,2,6,8,5,3,7,9,1], 
              [7,1,3,9,2,4,8,5,6],
              [9,6,1,5,3,7,2,8,4],
              [2,8,7,4,1,9,6,3,5],
              [3,4,5,2,8,6,1,7,8]]

if validateSudoku():
	print("Sudoku is valid.")
else:
	print("Sudoku is not valid.")

Output:

Sudoku is valid.

You can write a code in any other programming languages like C/C++, Java, etc.

Complexity

We are traversing sudoku elements with two for loops. The time complexity of this problem is 2*O(n) which is equivalent to O(n). This is the most optimal solution for solving this problem.

We are not taking any extra space, so the space complexity is O(1) i.e. constant space.

If you have any doubt or want to suggest any solution for valid sudoku checker, write me in the comment section.

Python Interview Questions eBook

coding challenge
Aniruddha Chaudhari
I am complete Python Nut, love Linux and vim as an editor. I hold a Master of Computer Science from NIT Trichy. I dabble in C/C++, Java too. I keep sharing my coding knowledge and my own experience on CSEstack.org portal.

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Why?

Why Competitive Programming Important?

Coding Challenges for Practice

  1. Count Common Factor
  2. Does it Divide
  3. Sum of Sub Arrays
  4. Pair of Desired Sum
  5. Remove Duplicate Char from String
  6. Sort String by Char Freq (Python)
  7. Sort String by Char Freq (Java)
  8. Split Array into Equal Sum Subarray
  9. Validate IP Address
  10. Validate PAN Card Number
  11. Validate Sudoku
  12. Sort Circular Rotated Array
  13. Min Arrow to Burst Bubbles
  14. Min Cost to Paint All Houses [Amazon]
  15. HourGlass with Largest Sum
  16. Max Profit by Buying/Selling Stocks
  17. Hailstone Sequence
  18. Reverse String without affecting Special Characters
  19. Secure Conversation by Encry/Decry
  20. Special Elements in Matrix
  21. Next Greater No with Same set of Digits
  22. Smallest Subarray with Sum Greater than Given Number
  23. Group Anagrams
  24. Find Duplicates in Array in O(n)
  25. Find Two Unique Numbers from Array in O(n)
  26. Number Patterns & Finding Smallest Number
  27. First Unique Element in a Stream
  28. Flip Equivalent Binary Trees [TeachMint]
  29. Minimum Cost of Merging Files [Amazon]
  30. Minimum Distance for Truck to Deliver Order [Amazon]
  31. Order Task for Given Dependencies
  32. Design Music Player
  33. Multilevel Parking System Design
  34. Minimum Coins Required
  35. Max Sum Subarray
  36. Max Avg Sum of Two Subsequences
  37. Merge Overlapping Intervals
  38. Longest Balanced Subarray
  39. Longest Path in a Weighted Tree
  40. Generate Balanced Parentheses
  41. PostOrder Traversal Without Recursion

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