[LeetCode] 227. Basic Calculator II

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

Use stack to postpone the operator evaluation.

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

Edited on 12/24/2021. Updated for a simpler stack based solution.