LeetcodeEvaluate Reverse Polish Notation

Posted 2020-07-16 wuezs

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

题目：

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

思路：

easy

算法：

public int evalRPN(String[] tokens) {  
    Stack<String> stack = new Stack<String>();  
  
    for (String token : tokens) {  
        stack.push(token);  
        if (!Character.isDigit(stack.peek().charAt(stack.peek().length() - 1))) {  
            String op = stack.pop();  
            String num2 = stack.pop();  
            String num1 = stack.pop();  
            String result = eval(op, num1, num2);  
            stack.push(result);  
        }  
  
    }  
    return Integer.parseInt(stack.pop());  
}  
  
private String eval(String op, String num1, String num2) {  
    int res = 0;  
    int n1 = Integer.parseInt(num1);  
    int n2 = Integer.parseInt(num2);  
  
    switch (op) {  
    case "+":  
        res = n1 + n2;  
        break;  
    case "-":  
        res = n1 - n2;  
        break;  
    case "*":  
        res = n1 * n2;  
        break;  
    case "/":  
        res = n1 / n2;  
        break;  
    }  
    return res + "";  
}