diff --git a/data_structures/arrays/sudoku.rb b/data_structures/arrays/sudoku.rb
index 2f960a8..9a6b3ae 100644
--- a/data_structures/arrays/sudoku.rb
+++ b/data_structures/arrays/sudoku.rb
@@ -160,6 +160,124 @@ print(is_valid_sudoku(board))
 # => false
 # explanation: duplicated value in row
 
+board = [["8","3",".",".","7",".",".",".","."],
+        ["6",".",".","1","9","5",".",".","."],
+        [".","9","8",".",".",".",".","6","."],
+        [".",".",".",".","6",".",".",".","3"],
+        ["4",".",".","8",".","3",".",".","1"],
+        ["7",".",".",".","2",".",".",".","6"],
+        [".","6",".",".",".",".","2","8","."],
+        [".",".",".","4","1","9",".",".","5"],
+        [".",".",".",".","8",".",".","7","9"]]
+print(is_valid_sudoku(board))
+# => false
+# explanation: duplicated value in 3x3 grid
+
+#
+# Approach 2: Sets
+#
+require 'set'
+
+def is_valid_sudoku(board)
+  return false unless check_rows(board)
+  return false unless check_cols(board)
+  row = -3
+  while (row += 3) < 9
+    col = - 3
+    while (col += 3) < 9
+      start_point = [row, col]
+      return false unless check_grid(board, start_point)
+    end
+  end
+  true
+end
+
+def check_grid(board, start_point)
+  row = start_point[0]
+  col = start_point[1]
+  ss = Set.new
+  (row..(row + 2)).each do |cur_row|
+    (col..(col + 2)).each do |cur_col|
+      next if board[cur_row][cur_col] == "."
+      return false if ss.member?(board[cur_row][cur_col])
+      ss.add board[cur_row][cur_col]
+    end
+  end
+  true
+end
+
+def check_col(board, col)
+  ss = Set.new
+  (0..8).each do |row|
+    next if board[row][col] == "."
+    return false if ss.member?(board[row][col])
+    ss.add board[row][col]
+  end
+  true
+end
+
+def check_row(board, row)
+  ss = Set.new
+  (0..8).each do |col|
+    next if board[row][col] == "."
+    return false if ss.member?(board[row][col])
+    ss.add board[row][col]
+  end
+  true
+end
+
+def check_rows(board)
+  (0..8).each do |row|
+    return false unless check_row(board, row)
+  end
+  true
+end
+
+def check_cols(board)
+  (0..8).each do |col|
+    return false unless check_col(board, col)
+  end
+  true
+end
+
+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"]]
+print(is_valid_sudoku(board))
+# => true
+
+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"]]
+print(is_valid_sudoku(board))
+# => false
+# explanation: duplicated value in column
+
+board = [["8","3",".",".","7",".","3",".","."],
+        ["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"]]
+print(is_valid_sudoku(board))
+# => false
+# explanation: duplicated value in row
+
 board = [["8","3",".",".","7",".",".",".","."],
         ["6",".",".","1","9","5",".",".","."],
         [".","9","8",".",".",".",".","6","."],