Clever solution from https://discuss.leetcode.com/topic/22359/16-ms-solution-in-c-with-stacks using awesome stacks!
For instance, 1+2+(3+4) as a case, we use sum to represent the sum, and we can calculate 1+2 to 3 simply, then we encounter (, we just put 3 to a stack named numStack, and put + to a stack named symStack (the + in front of (), after calculating the sum 3+4 until meeting ), we do a pop operation to the two stacks, and get 3+7 as a result (3 is on the top of the stack numStack, and + is on the top of the stack symStack).
There is two tricks in the code, clever tricks!
If the character is not a digit or (, we should do an add operation, but as we have set num to 0 after encountering ()+-, so we can do an add operation even when the character is (, for it will always get 0 to add up.
There is another trick when dealing with the result, when the string is 1+2, we should add up sign * num in the end, but if the string is like 1+(2), when doing the pop operations, we have added it to ans, so ans is the one should be returned. How to deal with the two cases? Don't worry, we set num to 0 after encountering ()+- like above, so we can always return ans + sign * num as the answer, for num will be 0 if it should be 0!
The clever solution is here, and if you are free, my ugly solution is there.