title: Golang question Array sudoku - grid[i][j][0]
body: <p>Hi I have a doubt with this code exactly with this <code>seenRow[grid[i][j][0] - 49]</code>. what does this ?</p>
  
  <pre class="lang-golang prettyprint-override"><code>
  func sudoku2(grid [][]string) bool {
      for i := 0; i &lt; 9; i++ {
          var seenRow [9]bool
          var seenCol [9]bool
          var seenSub [9]bool
          for j := 0; j &lt; 9; j++ {
              if grid[i][j] != "." {
                  if seenRow[grid[i][j][0] - 49] {
                      return false
                  }
                  seenRow[grid[i][j][0] - 49] = true
              }
              if grid[j][i] != "." {
                  if seenCol[grid[j][i][0] - 49] {
                      return false
                  }
                  seenCol[grid[j][i][0] - 49] = true
              }
              x := i%3*3 + j%3
              y := i/3*3 + j/3
              if grid[y][x] != "." {
                  if seenSub[grid[y][x][0] - 49] {
                      return false
                  }
                  seenSub[grid[y][x][0] - 49] = true
              }
          }
      }
      return true
  }
  
  </code></pre>