Implement a basic calculator to evaluate a simple expression string.

The expression string contains onlynon-negativeintegers,+,-,*,/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

Note:

• You may assume that the given expression is always valid.
• Do not use the eval built-in library function.

class Solution {
    public int calculate(String s) {
        if (s == null || s.length() == 0) return 0;
        s = s.trim();
        s = s.replaceAll("\\s", "");

        int num = 0;
        char sign = '+';
        Stack<Integer> stack = new Stack<>();
        for (int i = 0; i < s.length(); i++){
            char c = s.charAt(i);
            if (Character.isDigit(c)){
                num = num * 10 + (c - '0');
            }
            if (!Character.isDigit(c) || 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 = c;
                num = 0;
            }
        }

        int ans = 0;
        while (!stack.isEmpty()){
            ans += stack.pop();
        }

        return ans;
    }
}