Evaluate Reverse Polish Notation

Description

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Note:

• Division between two integers should truncate toward zero.
• The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.

Example 1:

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

Example 2:

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

Example 3:

Input: ["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

Solution(javascript)

/**
 * @param {string[]} tokens
 * @return {number}
 */
const evalRPN = function (tokens) {
  const stack = []
  const isOperator = token => token === '+' || token === '-' || token === '*' || token === '/'
  for (const token of tokens) {
    if (isOperator(token)) {
      const b = stack.pop()
      const a = stack.pop()
      let result = 0
      switch (token) {
        case '+':
          result = a + b
          break
        case '-':
          result = a - b
          break
        case '/':
          result = a / b
          result = result < 0 ? Math.ceil(result) : Math.floor(a / b) // 注意负数向上取整
          break
        default:
          result = a * b
      }
      stack.push(result)
    } else {
      stack.push(parseInt(token, 10))
    }
  }
  return stack[0]
}