R Apply function with if statements to every elements in list

I have a huge list, below is a sample of trboot6

$ 1  : num [1:39] -1 1 -1 -1 -1 -1 -1 -1 -1 1 ...
$ 2  : num [1:46] -1 -1 -1 1 1 1 -1 -1 -1 -1 ...
$ 3  : num [1:48] 1 -1 -1 -1 -1 -1 -1 -1 -1 -1 ...
$ 4  : num [1:43] -1 -1 1 -1 -1 -1 -1 -1 -1 -1 ...

What I want to do is check if in each list every pair has 1 and -1. Pairs are represented in brackets in the following:

$ 1  : num [1:39] (-1 1) (-1 -1) (-1 -1) (-1 -1) (-1 1) ...
$ 2  : num [1:46] (-1 -1) (-1 1) (1 1) (-1 -1) (-1 -1) ...
$ 3  : num [1:48] (1 -1) (-1 -1) (-1 -1) (-1 -1) (-1 -1) ...
$ 4  : num [1:43] (-1 -1) (1 -1) (-1 -1) (-1 -1) (-1 -1) ...

If the pair does not have 1 and -1 then, I want to delete the second same number, that is if the pair is (1 1), I delete the second 1. If there is 1 again, I delete this 1 too. Then if there is a -1, it will pair with the first 1.

To better code, I used the logic that the sum should always remain between -2 and 2 for the pairs to exist. Pair cant be (1,-1) (-1,1) or (1,-1) (1,-1). So if the balance goes <-2 or >2, the latest number has to be deleted.

Here is my code for the above logic:

balboot<-0
fboot<- function(x) {
  if(x==-1){balboot<-balboot-1}
  if(x==1){balboot<-+1}
  if(balboot==-2){x<-0 
  balboot=-1} 
  if(balboot==2){x<-0 
  balboot=1}
  return(fboot)
}
rdtp<-lapply(trboot6, FUN=fboot)

After running this, I get the warning:

In if (x == 1) { ... :
  the condition has length > 1 and only the first element will be used

thank you for your help in advance.