Sudoku Solver
Input requirements:

1. String must be strictly of size 81.
2. Denote an empty cell as 0 or ., everything else as {1,2,...,9}
3. Try some examples:
a. 530070000600195000098000060800060003400803001700020006060000280000419005000080079 (valid)
b. 531070000600195000098000060800060003400803001700020006060000280000419005000080079 (invalid)
c. .....7..9.4..812.....9...1...53...72293....5......53..8...23...7...5..4.531.7.... (valid)
d. 000007009040081200000900010005300072293000050000005300800023000700050040531070000 (valid)
e. 000000000000000000000000000000000000000000000000000000000000000000000000000000000 (valid)