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# 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.

```java
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;
    }
}
```