150. Evaluate Reverse Polish Notation（js）

150. Evaluate Reverse Polish Notation

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
题意：运算一个表达式，给定一个数组，操作符紧跟数字后面
代码如下：

/**
 * @param {string[]} tokens
 * @return {number}
 */
var evalRPN = function(tokens) {
    let option=[]
    if(tokens.length===1) return tokens.pop();
    for(let i=0;i<tokens.length;i++){
        let num1,num2;
        if(tokens[i]!=='+' && tokens[i]!=='-' && tokens[i]!=='*' &&   tokens[i]!=='/'){
            option.push(tokens[i]);            
        }else{
            num1=parseInt(option.pop());
            num2=parseInt(option.pop());
            switch(tokens[i]){
                case '+':
                    option.push(num2+num1)
                    break;
                case '-':
                    option.push(num2-num1)
                    break;
                case '*':
                    option.push(num2*num1)
                    break;
                case '/':
                    option.push(parseInt(num2/num1))
                    break;
            }
            
        }
    
    }
    return option.pop();
};