数据结构与算法之栈

leetcode 的题目：
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

解题思路就是利用栈的先进后出的特性

import java.util.*;
public class Solution {
public int evalRPN(String[] tokens) {
if(tokens == null){
return -1;
}

    Stack<Integer> stack=new Stack();   for(int i=0;i<tokens.length;i++){        try{           int num=Integer.parseInt(tokens[i]);           stack.add(num);        }catch(Exception e){            int a=stack.pop();            int b=stack.pop();            stack.push(getIt(a,b,tokens[i]));        }    }    return stack.pop();}public static int getIt(int a,int b,String str){    switch(str){        case "+":            return b+a;        case "-":            return b-a;        case "/":            return b/a;        case "*":            return b*a;        default :            return 0;    }}

}