227. Basic Calculator II

Problem:

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.

Example 1:

Input: "3+2*2"
Output: 7

Example 2:

Input: " 3/2 "
Output: 1

Example 3:

Input: " 3+5 / 2 "
Output: 5

Analysis:
Very similar to Basic Calculator I. The idea is to calculate the temp result whenever a sign is met and push into stack. If the prev sign is * or /, grab the stack top calc together.
For instance 3 + 2* 2.
+: calc +3 push into stack
*: calc +2 push into stack
string end: calc 2*2 push into stack

Solution:
Beware that the number might over 1 digit.

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