1. 去掉左边的或者右边的，将both改为left或right
    1. > library(stringr)
    2. >  a <- c('      aa bbcc    ddee   ','      aa bbcc    ddee   ','      aa bbcc    ddee   ')
    3. > str_trim(a,'both')
    4. [1] "aa bbcc    ddee" "aa bbcc    ddee" "aa bbcc    ddee"