根据r中每行中的日期序列，使用列名序列创建新列

我有一个数据框，其中包含几个日期列，我需要返回一个列名序列，其中包含每行从最早到最旧的日期.

即，如果起始数据是:

data.frame(ID = c(1,2,3,4,5,6),col1 = as.Date(c("2011-02-01","2020-09-01",NA,"2010-06-01",NA,"2015-02-02")),col2 = as.Date(c("2010-02-01","2018-09-01","2012-03-01",NA,NA,NA)), col3 = as.Date(c("1998-09-02",NA,"2013-06-01","2003-05-01","2000-02-01",NA)),col4=as.Date(c("2020-04-01",NA,"2019-09-10","2014-03-05",NA,"2000-08-12")))

  ID       col1       col2       col3       col4
1  1 2011-02-01 2010-02-01 1998-09-02 2020-04-01
2  2 2020-09-01 2018-09-01         NA         NA
3  3         NA 2012-03-01 2013-06-01 2019-09-10
4  4 2010-06-01         NA 2003-05-01 2014-03-05
5  5         NA         NA 2000-02-01         NA
6  6 2015-02-02         NA         NA 2000-08-12

那么最终的结果应该是:

data.frame(ID = c(1,2,3,4,5,6),col1 = as.Date(c("2011-02-01","2020-09-01",NA,"2010-06-01",NA,"2015-02-02")),col2 = as.Date(c("2010-02-01","2018-09-01","2012-03-01",NA,NA,NA)), col3 = as.Date(c("1998-09-02",NA,"2013-06-01","2003-05-01","2000-02-01",NA)),col4=as.Date(c("2020-04-01",NA,"2019-09-10","2014-03-05",NA,"2000-08-12")),seq = c("col3_col2_col1_col4","col2_col1","col2_col3_col4","col3_col1_col4","col3","col4_col1"))

ID       col1       col2       col3       col4                 seq
1  1 2011-02-01 2010-02-01 1998-09-02 2020-04-01 col3_col2_col1_col4
2  2 2020-09-01 2018-09-01         NA         NA           col2_col1
3  3         NA 2012-03-01 2013-06-01 2019-09-10      col2_col3_col4
4  4 2010-06-01         NA 2003-05-01 2014-03-05      col3_col1_col4
5  5         NA         NA 2000-02-01         NA                col3
6  6 2015-02-02         NA         NA 2000-08-12           col4_col1