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 # Sudoku
+Sudoku is a game with few simple rules, where the goal is to place nine sets of positive digits (1...9) into the cells of a fixed grid structure (i.e. board).
+
+The Sudoku board (or global grid) consists of a 3x3 arrangement of sub-grids, and each sub-grid is a 3x3 arrangement of cells. This yields a 9x9 arrangement of cells on the Sudoku board.
+
+A valid Sudoku solution should conform to the following rules:
+
+- R1: A cell in a Sudoku game can only store positive digits, i.e. 1...9.
+- R2: All digits appear only once in a sub-grid, i.e. they cannot repeat.
+- R3: A digit can appear only once in the rows of the global grid.
+- R4: A digit can appear only once in the columns of the global grid.
+Your task is to check the validity of a given solution for a Sudoku game
+
+You should read the candidate solution from a string variable, which should be exactly 81 characters long, i.e. first 9 are the first row, second 9 are the second row etc.  
+You shall check whether the provided string follows the correct format (i.e. 9 rows with 9 entries in each row).  
+You shall check the validity of the candidate solution against the rules listed above.  
+You program shall provide the functionality to return an integer code on the validity of the solution:
+- Return 0: means it is a valid Sudoku solution
+- Return -1: means it is violating Rule #1
+- Return -2: means it is violating Rule #2
+- Return -3: means it is violating Rule #3
+- Return -4: means it is violating Rule #4
+
+You should implement the functionality with a single API method with the following signature:  
+`public int verify(String candidateSolution)`
+
+A correct Sudoku string: 417369825632158947958724316825437169791586432346912758289643571573291684164875293
+
+An incorrect Sudoku string: 123456789912345678891234567789123456678912345567891234456789123345678912234567891