Given a string `s`

which represents an expression, *evaluate this expression and return its value*.

The integer division should truncate toward zero.

**Note: **You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as `eval()`

.

**Example 1:**

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

**Example 2:**

Input:s = " 3/2 "Output:1

**Example 3:**

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

**Constraints:**

`1 <= s.length <= 3 * 10`

^{5}`s`

consists of integers and operators`('+', '-', '*', '/')`

separated by some number of spaces.`s`

represents**a valid expression**.- All the integers in the expression are non-negative integers in the range
`[0, 2`

.^{31}- 1] - The answer is
**guaranteed**to fit in a**32-bit integer**.

class Solution {
public int calculate(String s) {
}
}