Basic Calculator II

https://leetcode.com/problems/basic-calculator-ii/description/

class Solution {
    public int calculate(String s) {
        if(s == null || s.length() == 0){
            return 0;
        }

        int len = s.length();
        char sign = '+';
        int num = 0;
        Deque<Integer> stack = new LinkedList<>();
        for(int i = 0; i < len; i++){
            char cur = s.charAt(i);
            if(Character.isDigit(cur)){
                num = num * 10 + cur - '0';
            }

            // && precedence > ||
            // When i == len - 1, the last value has not been added to the stack
            // also the loop will stop, so we need to consider this corner case
            if(!Character.isDigit(cur) && cur != ' ' || i == len - 1){
                if(sign == '+'){
                    stack.push(num);
                }
                if(sign == '-'){
                    stack.push(-num);
                }
                if(sign == '*'){
                    stack.push(stack.pop() * num);
                }
                if(sign == '/'){
                    stack.push(stack.pop() / num);
                }
                sign = cur;
                num = 0;
            }
        }

        int res = 0;
        for(Integer i : stack){
            res += i;
        }

        return res;
    }
}

Use stack to store the previous value for multiply and divide operations.