c++ 字符及字符串

a.length() = a.size() 返回string的字符个数
a.push_back('x') 末尾加字符x
a+=b =a.append(b) 字符串拼接
transform(a.begin(),a.end(),a.begin(),::tolower) 大写转小写
transform(a.begin(),a.end(),a.begin(),::toupper) 小写转大写
tolower('X')='x'
toupper('x')='X'