Leetcode 772. Basic Calculator III

Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

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

You may assume that the given expression is always valid. All intermediate results will be in the range of [-2147483648, 2147483647].

Some examples:

"1 + 1" = 2
" 6-4 / 2 " = 4
"2*(5+5*2)/3+(6/2+8)" = 21
"(2+6* 3+5- (3*14/7+2)*5)+3"=-12

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

Code (Java):

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

        // remove leading and trailing spaces and white spaces.
        //
        s = s.trim().replaceAll("[ ]+", "");

        if (s == null || s.length() == 0) {
            return 0;
        }

        Stack<Character> opStack = new Stack<>();
        Stack<Integer> numStack = new Stack<>();

        int i = 0;
        while (i < s.length()) {
            if (Character.isDigit(s.charAt(i))) {
                int num = 0;
                while (i < s.length() && Character.isDigit(s.charAt(i))) {
                    num = num * 10 + Character.getNumericValue(s.charAt(i));
                    i++;
                }
                numStack.push(num);
            } else {
                char op = s.charAt(i);
                if (opStack.isEmpty()) {
                    opStack.push(op);
                    i++;
                } else if (op == '+' || op == '-') {
                    char top = opStack.peek();
                    if (top == '(') {
                        opStack.push(op);
                        i++;
                    } else {
                        calculate(numStack, opStack);
                    }
                } else if (op == '*' || op == '/') {
                    char top = opStack.peek();
                    if (top == '(') {
                        opStack.push(op);
                        i++;
                    } else if (top == '*' || top == '/') {
                        calculate(numStack, opStack);
                    } else if (top == '+' || top == '-') {
                        opStack.push(op);
                        i++;
                    }
                } else if (op == '(') {
                    opStack.push(op);
                    i++;
                } else if (op == ')') {
                    while (opStack.peek() != '(') {
                        calculate(numStack, opStack);
                    }
                    opStack.pop();
                    i++;
                }
            }
        }

        while (!opStack.isEmpty()) {
            calculate(numStack, opStack);
        }

        return numStack.peek();
    }
    
    private void calculate(Stack<Integer> numStack, Stack<Character> opStack) {
        int num2 = numStack.pop();
        int num1 = numStack.pop();
        
        char op = opStack.pop();
        
        int ans = 0;
        
        switch(op) {
            case '+':
                ans = num1 + num2;
            break;
            case '-':
                ans = num1 - num2;
            break;
            case '*':
                ans = num1 * num2;
            break;
            case '/':
                ans = num1 / num2;
            break;
        }
        
        numStack.push(ans);
    }
}