Evaluate Reverse Polish Notation

Evaluate Reverse Polish Notation

You are given an array of strings tokens that represents an arithmetic expression in Reverse Polish Notation (postfix notation).

Evaluate the expression and return an integer that represents the value of the expression.

Note that:

• The valid operators are '+', '-', '*', and '/'.
• Each operand may be an integer or another expression.
• The division between two integers always truncates toward zero.
• There will not be any division by zero.
• The input represents a valid arithmetic expression in reverse polish notation.
• The answer and all intermediate calculations can be represented in a 32-bit integer.

Examples

Example 1:

Input: tokens = ["2", "1", "+", "3", "*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9

Example 2:

Input: tokens = ["4", "13", "5", "/", "+"]
Output: 6
Explanation: (4 + (13 / 5)) = 4 + 2 = 6

Example 3:

Input: tokens = ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]
Output: 22
Explanation:
  ((10 * (6 / ((9 + 3) * -11))) + 17 + 5)
  = ((10 * (6 / -132)) + 17 + 5)
  = ((10 * 0) + 17 + 5)
  = 22

Constraints

• 1 <= tokens.length <= 10^4
• tokens[i] is either an operator ("+", "-", "*", "/") or an integer in the range [-200, 200].

Expected Complexity

• Time: O(n)
• Space: O(n)