# [Whitespace], 67 bytes
This program requires a null byte to mark the end of input as Whitespace has no way to detect when stdin is empty. If you're using the TIO link make sure not to delete the last character in the input field (looks like a space) as it's a null byte. If you do you'll need to append a null byte using your browsers console.
<pre><code>   
  	
 
 	
	  
 			
	 
   	
	   
 
	
  
   	
	  	 
 				
  
 
</code></pre>
[Try it online!]
## Explanation
(s - space, t - tab, n - newline)
sssn ; push 0 to use as the starting heap address
nsstn ; label 'read-loop'
sns ; dup
tnts ; getchar and store at address n
sns ; dup
ttt ; retrieve the character value we just read
ntsn ; jez 'output-loop' - if it was a null byte switch to output
ssstn ; push 1
tsss ; add - increment n
nsntn ; jmp 'read-loop'
nssn ; label 'output-loop'
ssstn ; push 1
tsst ; sub - decrement n
sns ; dup
ttt ; retrieve the character value at address n
tnss ; putchar - display the character
nsnn ; jmp 'output-loop'
Whitespace uses a stack and a heap for data storage. As I/O commands write to the heap naturally we store the string on the heap and keep the stack for our counter.
This program reads characters one at a time and stores them in order starting from heap address 0 counting up. Once the program reads a null byte it starts displaying characters starting from the previous heap address counting down. For the input `hello\0` this populates the heap as `[#0:h,#1:e,#2:l,#3:l,#4:o,#5:\0]` then outputs the characters at addresses `#4,#3,#2,#1,#0` and we end up with `olleh`.
[Whitespace]: https://web.archive.org/web/20150618184706/http://compsoc.dur.ac.uk/whitespace/tutorial.php
[Try it online!]: https://tio.run/nexus/whitespace#PYqxCQAgEANtkymynKggWuj@77@CTXIcZ5JICRR9oDgEuHzCiYjmG7wI15FmNfc@ufPabZR0AA "Whitespace – TIO Nexus"