Postfix evaluation is a single left-to-right scan. Operands are pushed onto a value stack; when an operator appears, pop the top two values, apply the operator in the correct order, and push the result back. When the input is exhausted, the single value left on the stack is the answer — no parentheses and no precedence needed.
Postfix evaluation with a value stack4 2 3 * + →
push 4, push 2, push 3
*: pop 2, pop 3 → push 6
+: pop 4, pop 6 → push 10 → answer 10
For subtraction/division order matters: a b - means a − b.