improving ex3_19.cpp
improving ex3_19.cpp

wgs111/CppPrimer,FingerSport/CppPrimer,floristt/CppPrimer,pezy/CppPrimer,shuidong/CppPrimer,sheldon66/CppPrimer,at86/CppPrimer,zhangzhizhongz3/CppPrimer,m0000re/CppPrimer,zhangzhizhongz3/CppPrimer,akanezorap/CppPrimer,heamon7/CppPrimer,Milk-SF/CppPrimer,cseryp/CppPrimer,heamon7/CppPrimer,Sublele/CppPrimer,vzyw/CppPrimer,jwave/CppPrimer,jwave/CppPrimer,Milk-SF/CppPrimer,FingerSport/CppPrimer,shuidong/CppPrimer,laohei889/CppPrimer,lawlietye/CppPrimer,vzyw/CppPrimer,laohei889/CppPrimer,cheng668/CppPrimer,at86/CppPrimer,cseryp/CppPrimer,m0000re/CppPrimer,wgs111/CppPrimer,sheldon66/CppPrimer,lawlietye/CppPrimer,pezy/CppPrimer,Sublele/CppPrimer,floristt/CppPrimer,akanezorap/CppPrimer,cheng668/CppPrimer