leetcode中等150逆波兰表达式求值

Posted 2021-11-23 qq_40707462

思路：栈
遇到运算符，就去栈里找最上面两个数运算一下，再放回栈里

class Solution {
    public int evalRPN(String[] tokens) {
        Stack<Integer>stack=new Stack<>();
        for(String token:tokens){
            if(isNumber(token)) stack.push(Integer.parseInt(token));
            else{
                int num2 = stack.pop();
                int num1 = stack.pop();
                switch(token){
                    case "+":
                        stack.push(num1 + num2);
                        break;
                    case "-":
                        stack.push(num1 - num2);
                        break;
                    case "*":
                        stack.push(num1 * num2);
                        break;
                    case "/":
                        stack.push(num1 / num2);
                        break;
                    default:
                }
            }
        }
        return stack.pop();
    }
    public boolean isNumber(String token) {
        return !("+".equals(token) || "-".equals(token) || "*".equals(token) || "/".equals(token));
    }
}