public class Solution
extends Object
227 - Basic Calculator II\.
Medium
Given a string `s` which represents an expression, _evaluate this expression and return its value_.
The integer division should truncate toward zero.
You may assume that the given expression is always valid. All intermediate results will be in the range of [-231, 231 - 1].
**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 * 105
* `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, 231 - 1].
* The answer is **guaranteed** to fit in a **32-bit integer**.