In postfix notation (also called reverse Polish notation) an operator is written after the two values it works on: 3 + 4 becomes 3 4 +. No brackets are ever needed, because each operator acts on the two values produced just before it.
You are given such an expression as a list of strings, tokens. Each token is either a whole number or one of the four operators "+", "-", "*" and "/". Work out the value of the expression and return it.
8 2 - is 8 - 2./ is whole-number division that drops the fractional part, rounding toward zero: 7 / 2 is 3 and -7 / 2 is -3.* takes the two values just before it, 3 and 4, giving 12. Then + takes 2 and 12, giving 14. In ordinary notation this is 2 + 3 * 4.
7 2 - is 7 - 2 = 5, and then 5 4 * is 20: (7 - 2) * 4.
10 - 5 = 5, then 4 * 5 = 20. Last, 20 / -6 is -3.33…, and dropping the fraction gives -3. The token "-6" is a number; only a "-" on its own is the operator.
Constraints
1 <= tokens.length <= 10⁴"+", "-", "*", "/", or a whole number from -200 to 200 written in plain digits: a leading - for a negative number, never a + sign or a leading zero.tokens is a valid postfix expression: every operator has two values before it, and exactly one value is left at the end.Read the tokens left to right and keep the values not yet used on a stack. Each operator works on exactly the two most recent of them.
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