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# Validate a Sudoku
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## Definition
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Sudokus are number-puzzles in which cells in a 9×9 grid must be populated with the numbers `1` to `9`.
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The following constraints must apply to the 9×9 grid.
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* Each digit from `1` to `9` only occurs only once
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* per row
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* per column
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* per block (i.e. 3×3 sub grid)
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## Goal
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Validate, if a given Sudoku is a valid solution.
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The Sudoku must comply with the definition above.
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### Input
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A sequence of 81 digits, organized in 9 columns and 9 rows.
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The columns of a row are separated by blanks.
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Rows are separated by line-breaks.
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### Output
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A boolean value `true` or `false` defining the correctness of the input.
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### Constraints
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1 <= `digit` <= 9
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### Examples
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**Input:**
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```
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9 8 1 5 2 3 6 4 7
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6 3 4 8 7 9 2 5 1
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2 7 5 1 4 6 9 8 3
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1 9 6 4 8 7 5 3 2
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5 4 8 3 1 2 7 6 9
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7 2 3 6 9 5 4 1 8
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3 1 2 7 5 4 8 9 6
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4 6 9 2 3 8 1 7 5
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8 5 7 9 6 1 3 2 4
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```
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**Output:**
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`true`
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