【JAVA】【leetcode】【使用堆栈实现后向计算】

题目：evaluate-reverse-polish-notation

要求：

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

理解：

后向计算，如（2，1+3*），实为（2+1）*3；使用堆栈实现的原理就是push(2)进栈，push(1)进栈，当遇到符号“+”时，a = pop(),再b = pop(),实现b+a,

然后将(b+a)的和再push进栈，等到下一个符号的到来时再pop出栈参与计算，直到计算完输入的所有字符。

import java.util.*;
public class Solution {
    public int evalRPN(String[] tokens) {
        Stack<Integer> stack = new Stack<Integer>();
        for(int i=0;i<tokens.length;i++){
            try{
                int num = Integer.parseInt(tokens[i]);
                stack.push(num);
            }catch(Exception e){
                int a = stack.pop();
                int b = stack.pop();
                stack.push(operate(b,a,tokens[i]));
            }
        }
        return stack.pop();
    }     
        public int operate(int a,int b,String token){
            switch(token){
                case"+": return a+b;
                case"-": return a-b;
                case"*": return a*b;
                case"/":
                if(b==0)
                    return 0;
                else
                    return a/b;
                default:return 0;
            }
        }
        
}