LeetCode.150 Calculate Reverse Polish Notation

题目：

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

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

Some examples:

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

分析：

class Solution {    public int evalRPN(String[] tokens) {        //实现后缀表达式        //思路：使用stack存储数字，当遇到运算符时，就从stack中取出数字，之后把结果放回stack                if(tokens.length==0) return 0;        Stack<Integer> stack=new Stack<Integer>();        int firstNum=0,secNum=0;        for(int i=0;i<tokens.length;i++){            if(tokens[i].equals("+")){                firstNum=stack.pop();                secNum=stack.pop();                stack.push(firstNum+secNum);            }else if(tokens[i].equals("-")){                firstNum=stack.pop();                secNum=stack.pop();                stack.push(secNum-firstNum);            }else if(tokens[i].equals("*")){                firstNum=stack.pop();                secNum=stack.pop();                stack.push(firstNum*secNum);            }else if(tokens[i].equals("/")){                firstNum=stack.pop();                secNum=stack.pop();                stack.push(secNum/firstNum);            }else{                //只是数字,直接入栈                stack.push(Integer.parseInt(tokens[i]));            }        }                return stack.pop();            }}