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:

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

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

O(n) time O(n) space solution

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public class Solution {
    public int calculate(String s) {
        if (s == null || s.length() == 0) return 0;
        Stack<Integer> stack = new Stack<>();
        char sign = '+';
        int num = 0;
        
        for (int i = 0; i < s.length(); i++) {
            if (Character.isDigit(s.charAt(i))) {
                num = num * 10 + s.charAt(i) - '0';
            }
            if ((!Character.isDigit(s.charAt(i)) && s.charAt(i) != ' ') || i == s.length() - 1) {
                if (sign == '+') {
                    stack.push(num);
                } else if (sign == '-') {
                    stack.push(-num);
                } else if (sign == '*') {
                    stack.push(stack.pop() * num);
                } else if (sign == '/') {
                    stack.push(stack.pop() / num);
                }
                sign = s.charAt(i);
                num = 0;
            }
        }
        int res = 0;
        for (int i : stack) {
            res += i;
        }
        return res;
    }
}