150-evaluate-reverse-polish-notation

150. Evaluate Reverse Polish Notation (Medium) (https://leetcode.com/problems/evaluate-reverse-polish-notation)

You are given an array of strings tokens that represents an arithmetic expression in a Reverse Polish Notation. Evaluate the expression. 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 a reverse polish notation. • The answer and all the intermediate calculations can be represented in a 32-bit integer.

const ops: Record<string, (a: number, b: number) => number> = {
  '+': (a, b) => a + b,
  '-': (a, b) => a - b,
  '*': (a, b) => a * b,
  '/': (a, b) => Math.trunc(a / b),
}
function evalRPN(tokens: string[]): number {
  const stack: number[] = []
 
  for (const token of tokens) {
    if (token in ops) {
      const right = stack.pop()!
      const left = stack.pop()!
      stack.push(ops[token](left, right))
    }
    else {
      stack.push(Number(token))
    }
  }
 
  return stack[0]
}
 
// Local check:
console.log(evalRPN(['2', '1', '+', '3', '*'])) // 9
console.log(evalRPN(['4', '13', '5', '/', '+'])) // 6
console.log(evalRPN(['10', '6', '9', '3', '+', '-11', '*', '/', '*', '17', '+', '5', '+'])) // 22

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)) = 6
 
  Example 3:
 
    Input: tokens = ["10","6","9","3","+","-11","*","/","*","17","+","5","+"]
    Output: 22
    Explanation: ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
    = ((10 * (6 / (12 * -11))) + 17) + 5
    = ((10 * (6 / -132)) + 17) + 5
    = ((10 * 0) + 17) + 5
    = (0 + 17) + 5
    = 17 + 5
    = 22

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