Evaluate Token Expression

You are given an array of strings tokens describing an arithmetic expression in postfix form: every operator comes after the two values it applies to, so the expression needs no parentheses.

Return the value of the expression.

• The valid operators are "+", "-", "*" and "/"; every other token is an integer.
• An operator applies to the two most recently produced values, in order: the value produced earlier is the left operand.
• Division between two integers truncates toward zero, so -7 / 2 is -3, not -4.
• The input is always a valid expression and never divides by zero.
• The answer and every intermediate value fit in a signed 32-bit integer.

Examples

Example 1

Input: tokens = ["2","1","+","3","*"]
Output: 9
Explanation: "+" applies to 2 and 1, giving 3. Then "*" applies to that 3 and the following 3, giving 9.

Example 2

Input: tokens = ["4","13","5","/","+"]
Output: 6
Explanation: "/" applies to 13 and 5, giving 2. Then "+" applies to 4 and 2, giving 6.

Example 3

Input: tokens = ["10","6","9","3","+","-11","*","/","*","17","+","5","+"]
Output: 22
Explanation: The expression reduces step by step to 22. Note that "-11" is an operand, not the operator "-".