Evaluate POSTFIX

For each symbol:
    If it is a number, push it.
    If it is a symbol, pop two numbers, operate, and push the result back on the stack.

Evaluate INFIX

For each number or symbol in the input:
    If it is a number, push it onto the numbers stack.
    If it is a symbol, then...
        If the symbol stack is empty, push the symbol onto the symbol stack.
        Else if the new symbol has higher precedence than the top of the symbol stack,
           then push the new symbol to the symbol stack.
        Else
            Pop a symbol, pop two numbers, operate, and push the result.
            Repeat until the symbol stack is empty or has an operator of lower or same
              precedence

While the symbol stack is not empty:
    Pop a symbol, pop two numbers, operate, and push the result.

Answer is the sole number on the numbers stack.