create an empty stack
create an empty list to represent the postfix expression

for each token in the expression:
    if token is a number:
        append it onto the postfix expression

    elif token is a left parenthesis:
        push it onto the stack

    elif token is an operator:
        while (stack is not empty and the top stack item is an
               operator with precedence greater than or equal to token):
            pop and append the operator onto the postfix expression
        push the token onto the stack

    else token must be a right parenthesis:
        while the top item on the stack is not a left parenthesis:
            pop item from the stack and append it onto the postfix expression
        pop the left parenthesis

while the stack is not empty:
    pop an item from the stack and append it onto the postfix expression