Fix bugs according to last submit.
Change some style. And fix a bug in Board::twin.

There is some bug in Board::twin which need to fix.

Submit result:

Compilation:  PASSED
Style:        PASSED
Findbugs:     No potential bugs found.
API:          PASSED

Correctness:  23/25 tests passed
Memory:       8/8 tests passed
Timing:       0/17 tests passed

Raw score: 69.80% [Correctness: 65%, Memory: 10%, Timing: 25%, Style: 0%]