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; } } }
原文：http://www.cnblogs.com/weimiaomiao/p/5847145.html