leetcode 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

1,放在堆里面做，碰到符号类就pop 两个数
2，记得输入可能有负数的情况，

tokens[i].matches("-?\d+")
3，string 不能用＝＝ 必须采用equal

public class Solution {
    public int evalRPN(String[] tokens) {
        Stack<Integer> store=new Stack<Integer>();
        int sum=0;
        for(int i=0;i<tokens.length;i++){
        	if(!tokens[i].matches("-?\d+")){
        		int a=store.pop();
        		int b=store.pop();
          		if(tokens[i].equals("+")){
        			sum=b+a;
        		}
        		else if(tokens[i].equals("-")){
        			sum=b-a;
        		}
        		else if(tokens[i].equals("*")){
        			sum=b*a;
        		}
        		else if(tokens[i].equals("/")){
        			sum=b/a;
        		}
        		store.push(sum);
        	}
        	else{
        		store.push(Integer.parseInt(tokens[i]));
        	}
        }
        return store.pop();
    }
}