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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.

You may assume that the given expression is always valid.

Some examples:

"3+2*2" = 7 " 3/2 " = 1 " 3+5 / 2 " = 5

**Note:** **Do not** use the `eval`

built-in library function.

**Credits:**

Special thanks to @ts for adding this problem and creating all test cases.

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