LeetCode Q227 Basic Calculator II

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.

You may assume that the given expression is always valid.

Some examples:

"3+2*2" = 7
" 3/2 " = 1
" 3+5 / 2 " = 5

Note: Do not use the eval built-in library function.

Solution:

The key is to "look back" when encounter a new operator. We compute results of "*" and "/" as we goes, and save all signed numbers and results in a stack. What left at the end is to only compute the sum of all elements in the stack.

Round 2 solution: