Print lines that have no duplicates in a file and preserve sort order linux

I have the following file:

2
1
4
3
2
1

I want the output like this (unique lines that don’t have any duplicates and preserve order):

4
3

I tried sort file.txt | uniq -u it works, but output is sorted:

3
4

I tried awk '!x[$0]++' file.txt it keeps order, but it prints all values once:

2
1
4
3

>Solution :

A couple ideas to choose from:

a) read the input file twice:

awk '
FNR==NR         { counts[$1]++; next }  # 1st pass: keep count
counts[$1] == 1                         # 2nd pass: print rows with count == 1
' file.txt file.txt

b) read the input file once (requires all rows to be stored in memory – via an array):

awk '
    { lines[NR] = $1                    # maintain ordering of rows
      counts[$1] ++
    }
END { for ( i=1;i<=NR;i++ )             # run thru the indices of the lines[] array and ...
          if ( counts[i] == 1 )         # if the associated count == 1 then ...
             print lines[i]             # print the array entry to stdout
    }
' file.txt

Both of these generate:

4
3