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Basic Calculator II

Medium · stack

A tiny calculator receives its sum as text. Given a string s holding an arithmetic expression, return its value.

The expression is made of non-negative whole numbers and the four operators +, -, * and /, with any number of spaces between them. There are no brackets.

• * and / are done before + and -.
• Operators of the same kind are done from left to right, so 8 - 3 - 2 is 3.
• / is whole-number division that rounds toward zero, so 7 / 2 is 3 and 0 - 7 / 2 is -3.

The expression is always valid, there is never a division by zero, and every value that appears while you work it out fits in a signed 32-bit integer.

Examples

Example 1

Input: s = "3+2*2"

Output: 7

The multiplication goes first: 2 * 2 = 4, then 3 + 4 = 7.

Example 2

Input: s = " 3/2 "

Output: 1

3 divided by 2 is 1.5, which rounds toward zero to 1.

Example 3

Input: s = " 3+5 / 2 "

Output: 5

5 / 2 is 2, and 3 + 2 is 5.

Example 4

Input: s = "0-7/2"

Output: -3

7 / 2 is 3, and 0 - 3 is -3.

Constraints

• 1 <= len(s) <= 3 * 10^5
• s contains digits, spaces and the characters '+', '-', '*' and '/'.
• s is a valid expression: numbers and operators alternate, it starts and ends with a number, and no number is split by a space.
• Every number is between 0 and 2^31 - 1, and every value that appears during evaluation fits in a signed 32-bit integer.
• There is no division by zero.