LeetCode-Evaluate Reverse Polish Notation

题目：https://oj.leetcode.com/problems/evaluate-reverse-polish-notation/

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：使用栈即可。遇到运算符，弹出两个操作数进行运算，把运算结果再压入栈；遇到操作数，直接进栈。最后把栈中最后一个元素（即计算结果）返回。

源码：Java版本
算法分析：时间复杂度O(n)

public class Solution {
    public int evalRPN(String[] tokens) {
        Stack<Integer> stack=new Stack<Integer>();
        int x,y;
        for(String str : tokens) {
            if(str.equals("+")) {
                stack.push(stack.pop()+stack.pop());
            }else if(str.equals("-")) {
                x=stack.pop();
                y=stack.pop();
                stack.push(y-x);
            }else if(str.equals("*")) {
                 stack.push(stack.pop()*stack.pop());
            }else if(str.equals("/")) {
                x=stack.pop();
                y=stack.pop();
                stack.push(y/x);
            }else {
                stack.push(Integer.parseInt(str));
            }
        }
        return stack.pop();
    }
}