逆波兰表达式

逆波兰表达式是用栈存取数字和运算符

规则就是

遇到运算符就从栈弹出两个数字，运算后把结果再存进去，不断进行

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

import java.util.Stack;
public class Solution {
    public int evalRPN(String[] tokens) {
        Stack<Integer> stack = new Stack();
        for(int i = 0;i < tokens.length;i++)
        {
            String cur = tokens[i];
            if(cur.length()>1||(cur.charAt(0)>='0'&&cur.charAt(0)<='9'))
                stack.push(Integer.parseInt(cur));
            else
            {
                int a = stack.pop();
                int b = stack.pop();
                if(cur.equals("+"))
                    stack.push(b+a);
                else if(cur.equals("*"))
                    stack.push(b*a);
                else if(cur.equals("-"))
                    stack.push(b-a);
                else if(cur.equals("/"))
                    stack.push(b/a);
            }
        }
        return stack.pop();
    }
}