valid-sudoku py3

0036_valid-sudoku/README.md

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+Determine if a `9 x 9` Sudoku board is valid. Only the filled cells need to be validated **according to the following rules**:
+
+1.  Each row must contain the digits `1-9` without repetition.
+2.  Each column must contain the digits `1-9` without repetition.
+3.  Each of the nine `3 x 3` sub-boxes of the grid must contain the digits `1-9` without repetition.
+
+**Note:**
+
+*   A Sudoku board (partially filled) could be valid but is not necessarily solvable.
+*   Only the filled cells need to be validated according to the mentioned rules.
+
+**Example 1:**
+
+![](https://upload.wikimedia.org/wikipedia/commons/thumb/f/ff/Sudoku-by-L2G-20050714.svg/250px-Sudoku-by-L2G-20050714.svg.png)
+
+    Input: board = 
+    [["5","3",".",".","7",".",".",".","."]
+    ,["6",".",".","1","9","5",".",".","."]
+    ,[".","9","8",".",".",".",".","6","."]
+    ,["8",".",".",".","6",".",".",".","3"]
+    ,["4",".",".","8",".","3",".",".","1"]
+    ,["7",".",".",".","2",".",".",".","6"]
+    ,[".","6",".",".",".",".","2","8","."]
+    ,[".",".",".","4","1","9",".",".","5"]
+    ,[".",".",".",".","8",".",".","7","9"]]
+    Output: true
+    
+
+**Example 2:**
+
+    Input: board = 
+    [["8","3",".",".","7",".",".",".","."]
+    ,["6",".",".","1","9","5",".",".","."]
+    ,[".","9","8",".",".",".",".","6","."]
+    ,["8",".",".",".","6",".",".",".","3"]
+    ,["4",".",".","8",".","3",".",".","1"]
+    ,["7",".",".",".","2",".",".",".","6"]
+    ,[".","6",".",".",".",".","2","8","."]
+    ,[".",".",".","4","1","9",".",".","5"]
+    ,[".",".",".",".","8",".",".","7","9"]]
+    Output: false
+    Explanation: Same as Example 1, except with the 5 in the top left corner being modified to 8. Since there are two 8's in the top left 3x3 sub-box, it is invalid.
+    
+
+**Constraints:**
+
+*   `board.length == 9`
+*   `board[i].length == 9`
+*   `board[i][j]` is a digit `1-9` or `'.'`.

0036_valid-sudoku/python3/solution.py

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+class Solution:
+    def isValidSudoku(self, board: List[List[str]]) -> bool:
+        #
+        # NOTE: **READ THE PROBLEM DESCRIPTION ALWAYS!!**
+        #
+        # Even though it mentions Sudoku, it doesn't mean the
+        # rules are the same as the real world. Which is the case
+        # here — we need to only look at the filled cell and the rules
+        # provided.
+        
+        size = len(board)
+        
+        # 1. Check if rows and cols are valid
+        #
+        # We can iterate both rows and cols at the same time since
+        # the sudoku board has 9 rows and 9 cols
+        for i in range(size):
+            row = set()
+            col = set()
+            
+            for j in range(size):
+                # ith row check
+                if board[i][j] != '.':
+                    if board[i][j] in row:
+                        return False
+                    
+                    row.add(board[i][j])
+                
+                # ith col check
+                if board[j][i] != '.':
+                    if board[j][i] in col:
+                        return False
+                    
+                    col.add(board[j][i])
+        
+        # 2. Check if squares are valid
+        # 
+        # We can do normal row * col iteration and use `// 3` to figure
+        # out which square a cell belongs to
+        
+        squares = [[set(), set(), set()] for _ in range(3)]
+        for i in range(size):
+            for j in range(size):
+                cell = board[i][j]
+                if cell == '.':
+                    continue
+                
+                square = squares[i // 3][j // 3]
+                if cell in square:
+                    return False
+                
+                square.add(cell)
+        
+        return True