CareerCup

Use two stacks, one for operands and one of operators. Whenever you notice parenthesis, take special action. Solution in Python below:

def evaluateInfix(expr):

  def helper(operator, x, y):
    if operator == '+': return x + y
    if operator == '-': return x - y
    if operator == '/': return y // x
    if operator == '*': return x * y

  operandStack = [] # for numbers
  operatorStack = [] # for arithmetic operators and parens.
  operators =  {
    '/': 1,
    '*': 1,
    '+': 2,
    '-': 2
  }
  for token in expr:

    if token.isdigit():
      operandStack.append(token)
    elif token == '(':
      operatorStack.append(token)

    elif token == ')':
      while operatorStack[-1] != '(':
        operandStack.append()

    elif token == ')':
      while operatorStack[-1] != '(':
        result = helper(operatorStack.pop(), int(operandStack.pop()), int(operandStack.pop()))
        operandStack.append(result)
      operatorStack.pop()

    elif token in operators:
      while len(operatorStack) and operators[operatorStack[-1]] <= operators[token]:
        result = helper(operatorStack.pop(), int(operandStack.pop()), int(operandStack.pop()))
        operandStack.append(result)

      operatorStack.append(token)

  while len(operatorStack):
    result = helper(operatorStack.pop(), int(operandStack.pop()), int(operandStack.pop()))
    operandStack.append(result)

  return operandStack.pop()

Test Code:

print(evaluateInfix('2 + 3 * 5')) # 17
print(evaluateInfix('4 * 5 / 2')) # 10