No150-Evaluate Reverse Polish Notation

Problem Description

Evaluate the value of an arithmetic expression in Reverse Polish Notation.
Valid operators are +, -, *, /. Each operand may be an integer or another expression.
Examples:

["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

Analyse

計算逆波蘭表達式求值，堆棧實現 javascript裏面是push and pop操作。有一個坑就是異號除法的取整問題。如果是負數那麼應該向上取整用ceil如果是正數應該向下取整。floor這一點要加個判斷

Method one: stack push and pop O(n)

JavaScript代碼如下:

var evalRPN = function(tokens) {
    var arr = new Array(tokens.length);
    if(tokens.length == 1) return parseInt(tokens[0]);
    var len = tokens.length;
    for(var i=0;i<len;i++){
        var m = tokens.shift();
        if(isNum(m)){
            arr.push(parseInt(m));
        }else{
            var b = arr.pop();
            var a = arr.pop();
            if(m == '*'){
                arr.push(a*b);
            }
            if(m == '+') arr.push(a+b);
            if(m == '-') arr.push(a-b);
            if(m == '/') {
                if(a/b>0) 
                arr.push(Math.floor(a/b));
                else arr.push(Math.ceil(a/b));

            }

        }
    }
    return arr.pop();
};
function isNum(str){
    if(str=='+'||str =='-'|| str =='*'||str =='/') return false;
    else return true;
}
};