使用非数字列手动排序数据框

i have the following data frame

我有以下数据框架

    dd <- data.frame(b = c("High", "Medium", "Highest", "Low", "Not bad","Good", "V. Good"),
   x = c("C3", "C1", "C4", "N2", "C2", "N1","N4"), x = c("5", "2", "3", "6", "7", "5","7") )