From Sudopedia, the free Sudoku reference guide
Validate
From Sudopedia
To determine that a sudoku has a unique solution, it needs to be validated. This is usually done with a backtracking algorithm, but it can also be done with a human-style solver, that does not use any uniqueness based techniques.
When a sudoku is validated with a backtracking algorithm, it only needs to find 2 solutions to declare the sudoku invalid. There is no need to count all solutions. Many programs do perform this count, but with a maximum. There could be billions of solutions to an invalid sudoku.
When a sudoku is validated with a human-style solver, the sudoku is declared invalid when no solution can be found. Thay may be true for the solver used, but if it does not handle every situation, a sudoku with only a single solution could be declared invalid.
