added 9.1
added 9.2
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
added 9.3 to 9.8, and split ex9_2_4_5_6.cpp
added 9.11 to 9.13
improved formatting for ex9_13.cpp
Merge remote-tracking branch 'Mooophy/master'
added 9.9 and 9.10
fixed 9.14
added 9.14 to 9.17

1. refactor of 9.14 and 9.16
2. separate ex9_15_16_17.cpp
fixed links of code
added 9.18 to 9.22

1. separate the ex9_18_19_20.cpp
2. refactor 9.19
improved some details
added 9.23 and improved 9.24
added 9.25 and fixed 9.26:should have a list...
added 9.27 and 9.28. deleted origin
added 9.29 and 9.30
refuctor ex9_31_32_34.cpp and added 9.33

1. 9.31 have an other case: forward_list
2. 9.32 it's not always illegal.
3. 9.34 infinite loop added fixed solution.
improved formatting for 9.38, added 9.35 to 9.40