A cool algorithm to check a Sudoku field?

Question

Does anyone know a simple algorithm to check if a Sudoku-Configuration is valid? The simplest algorithm I came up with is (for a board of size n) in Pseudocode

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Answer 1

Check each row, column and box such that it contains the numbers 1-9 each, with no duplicates. Most answers here already discuss this.

But how to do that efficiently? Answer: Use a loop like

result=0;
for each entry:
  result |= 1<<(value-1)
return (result==511);

Each number will set one bit of the result. If all 9 numbers are unique, the lowest 9 bits will be set. So the "check for duplicates" test is just a check that all 9 bits are set, which is the same as testing result==511. You need to do 27 of these checks.. one for each row, column, and box.