Initial commit
fix syntax error
solved ex  2.19
solved ex  2.20
solved ex  2.21
solved ex  2.23
solved Ex 2.27
solved Ex 2.28
solved Ex 2.29
solved Ex 2.30
solved Ex 2.31
solved Ex 2.32
solved Ex 2.33
solved Ex 2.34
solved Ex 2.35
solved Ex 2.36
solved Ex 2.37
solved Ex 2.40
solved N-queen problem
Merge branch 'initial'