Valid Sudoku

Solution.cs

using System;
using System.Collections.Generic;
using System.Linq;

public class Solution {
    public bool IsValidSudoku(char[][] board) {
      List<List<HashSet<int>>> sudokuSection = Enumerable.Range(1, 3)
        .Select(x => Enumerable.Range(1, 3).Select(x1 => new HashSet<int>(9)).ToList()).ToList();
      
      List<HashSet<int>> rows = Enumerable.Range(1, 9).Select(x => new HashSet<int>(9)).ToList();
      
      List<HashSet<int>> columns = Enumerable.Range(1, 9).Select(x => new HashSet<int>(9)).ToList();
      
      for(int r = 0; r < 9; r++)
      {
        for(int c = 0; c < 9; c++)
        {
          if(board[r][c] == '.') continue;
          var currentNum = board[r][c] - '0';
          
          if(currentNum >= 1 && currentNum <= 9)
          {
            if(!rows[r].Add(currentNum)) return false;
            if(!columns[c].Add(currentNum)) return false;
            
            if(!sudokuSection[r/3][c/3].Add(currentNum)) return false;
          }
          else
          {
            return false;
          }
        }
      }
      
      return true;
    }
}