Prefix Notation to Postfix Notation
brainfuck, 32 bytes

,[[-&gt;++++&lt;&lt;+&gt;]&gt;[[-]&lt;&lt;[.[-]&lt;]]&gt;,]


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I used @ as the operation, because its code point (64) is convenient.  U is also possible with the same byte count, using 3*85+1=256=0.

Explanation

The tape is used as a stack.  In each iteration of the main loop, the data pointer starts two cells right of the top of the stack.

,[                Take input and start main loop
  [-&gt;++++&lt;&lt;+&gt;]    Push input, and compute 4*input
  &gt;[              If 4*input is nonzero (and thus input is not @):
    [-]&lt;&lt;           Zero out this cell and move to top of stack
    [.[-]&lt;]         Pop from stack and output until \0 is reached
  ]
  &gt;,              Move pointer into the correct position.  If input was @, the earlier &gt; pushed \0 onto the stack.
]