Validates that sudoku grid contains once instance of each number per column and row.

sudoku_validator.py

def check_sudoku(grid):
    n = len(grid)
    digit = 1
    while digit <= n:
        i = 0
        while i < n:
            row_count = 0
            column_count = 0
            j = 0
            while j < n:
                if grid[i][j] == digit:
                    row_count = row_count + 1
                if grid[j][i] == digit:
                    column_count = column_count + 1
                j = j + 1
            if column_count != 1 or row_count != 1:
                return False
            i = i + 1
        digit = digit + 1   
    return True
    

In a 3x3 grid:

i = 0 , j = 0 and is incremented until same as length of grid

[0][0], [0][1], [0][2]
[0][0], [1][0], [2][0]

i is incremented by 1

i = 1 , j = 0 and is incremented until same as length of grid
[1][0], [1][1], [1][2]
[0][1], [1][1], [2][1]

i is incremented by 1

i = 2 , j = 0 and is incremented until same as length of grid
[2][0], [2][1], [2][2]
[0][2], [1][2], [2][2]