Java: Valid Sudoku

Difficulty Level: MEDIUM

Problem Statement:

Determine if a 9x9 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 3x3 sub-boxes of the grid must contain the digits 1-9 without repetition.

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