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(); } }