How do I reverse gather function in R?

I used gather function from tidyr to change the format of data wide to long. After that I removed NA values. How can I reverse this operation ?

first.df <- data.frame(a = c(1,NA,NA),b=c(2,3,NA),c=c(4,5,6)) 
second.df <- first.df %>% gather("x","y") %>% na.omit %>% `rownames<-`( NULL )

second.df:
|x|y|
|-|-|
|a|1|
|b|2|
|b|3|
|c|4|
|c|5|
|c|6|

How can I reverse this with spread or another function to first.df below ?

>Solution :

It requires a sequence column and then either dcast or pivot_wider can work or spread

library(dplyr)
library(tidyr)
library(data.table)
second.df %>% 
  mutate(rn = rowid(x)) %>% 
  pivot_wider(names_from = x, values_from = y) %>% 
  select(-rn)

-output

# A tibble: 3 × 3
      a     b     c
  <dbl> <dbl> <dbl>
1     1     2     4
2    NA     3     5
3    NA    NA     6

Or with dcast

library(data.table)
dcast(second.df, rowid(x) ~ x, value.var = 'y')[,-1]