150. Evaluate Reverse Polish Notation

Leetcode Stack

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Note:

• Division between two integers should truncate toward zero.
• The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.

Example 1:

Input: ["2", "1", "+", "3", "*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9

Example 2:

Input: ["4", "13", "5", "/", "+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6

Example 3:

Input: ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]
Output: 22
Explanation: 
  ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
= ((10 * (6 / (12 * -11))) + 17) + 5
= ((10 * (6 / -132)) + 17) + 5
= ((10 * 0) + 17) + 5
= (0 + 17) + 5
= 17 + 5
= 22

分析

这道题目直接用Stack就可以了。唯一需要注意的就是在做除法和减法的时候，注意前后两个数字的顺序。

/**
 * https://leetcode.com/problems/evaluate-reverse-polish-notation/description/
 * Evaluate the value of an arithmetic expression in Reverse Polish Notation.
 */
public class Q150EvaluateReversePolishNotation {
    public int evalRPN(String[] tokens) {
        Stack<Integer> numberStack = new Stack<>();
        Integer dummy;

        for (String token : tokens) {
            switch (token) {
                case "+":
                    numberStack.push(numberStack.pop() + numberStack.pop());
                    break;
                case "-":
                    numberStack.push(-numberStack.pop() + numberStack.pop());
                    break;
                case "*":
                    numberStack.push(numberStack.pop() * numberStack.pop());
                    break;
                case "/":
                    dummy = numberStack.pop();
                    numberStack.push(numberStack.pop() / dummy);
                    break;
                default:
                    numberStack.push(Integer.parseInt(token));

            }  // end switch
        } // end for
        return numberStack.pop();
    }
}