('+', '-', '*', '/') separated by some number of spaces.

s represents a valid expression.

All the integers in the expression are non-negative integers in the range [0, 2^31 - 1].

The answer is guaranteed to fit in a 32-bit integer.

Solution

We can proceed similar to Basic Calculator 2. But this time along with numStack, we take operator stack as well.

Each time we see digit, we make a number out of it , we push it numStack
If we see operator, we push it to opStack.
1. But before pushing it, we check if there are any higher precedence operator already in stack. If so, we should first process them, by poping them out and corresponding numbers from numStack
If we see ) , we just push it to opStack
If we see (, we should pop everything out from opStack until we see