diff --git a/work/1/Medium/Array/36.valid-sudoku.py b/work/1/Medium/Array/36.valid-sudoku.py
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+"""
+36. Valid Sudoku
+Difficulty: Medium
+https://leetcode.com/problems/valid-sudoku/
+
+──────────────────────────────────────────────────
+
+Determine if a 9 x 9 Sudoku board is valid. Only the filled cells
+need to be validated according to the following rules:
+
+        • Each row must contain the digits 1-9 without repetition.
+
+        • Each column must contain the digits 1-9 without repetition.
+
+• 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:
+
+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 '.'.
+"""
+
+import collections
+
+
+class Solution:
+    def isValidSudoku(self, board: list[list[str]]) -> bool:
+        cols = collections.defaultdict(set)
+        rows = collections.defaultdict(set)
+        squares = collections.defaultdict(set)  # (r/3, c/3)
+
+        for r in range(9):
+            for c in range(9):
+                if board[r][c] == ".":
+                    continue
+                if (
+                    board[r][c] in rows[r]
+                    or board[r][c] in cols[c]
+                    or board[r][c] in squares[(r // 3, c // 3)]
+                ):
+                    return False
+                cols[c].add(board[r][c])
+                rows[r].add(board[r][c])
+                squares[(r // 3, c // 3)].add(board[r][c])
+
+        return True