Basic Calculator II

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

Note:

• You may assume that the given expression is always valid.

• Do not use the eval built-in library function.

public class Solution {
    public int calculate(String s) {
        int len = s.length();
        if (s == null || len == 0)
            return 0;
        Stack<Integer> stack = new Stack<Integer>();
        int num = 0;
        char sign = '+';
        for (int i = 0; i < len; i++) {
            char x = s.charAt(i);
            if (x >= '0' && x <= '9')
                num = num * 10 + (x - '0');
            if ((x == '+' || x == '-' || x == '*' || x == '/') || 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 = x;
                num = 0;
            }
        }
        
        int ans = 0;
        for (int i : stack)
            ans += i;
        return ans;
    }
}