Description:

My fastest algorithm is #17 Worldwide on SPOJ (http://www.spoj.com/ranks/SUD/lang=JAVA)

Solves Sudokus with 3 approaches:

VERY SLOW:
1. Simple Backtracking (1..9)

SLOW:
2. Reduced Domain Backtracking (Only values which satisfy initial grid constraints)

VERY FAST:
3. Constraint Propagation Backtracking (Updates possibilities for each cell as we go)

My interest in solving Sudoku algorithmically was inspired by Peter Norvig's Python essay at: http://norvig.com/sudoku.html