[LeetCode#150]Evaluate Reverse Polish Notation

標籤：

Problem:

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

Analysis:

Reference: https://en.wikipedia.org/wiki/Reverse_Polish_notationAfter reading the wiki page of the RPN, you could find out this problem is super easy!!!!But I still stupidly make A mistake!!!-----------------------------------------------------------------------------------------Mistake 1:Forget to take care of the order, when numbers were poped out from stack. |num2||num1|Note: num2 was poped before num1!!!For an oprand (+,-,*,/), we hope num1 (operand) num2I ingore the order at first, thus I have implemented following wrong codes:if (token.equals("+") || token.equals("-") || token.equals("*") || token.equals("/")) {    int num1 = Integer.valueOf(stack.pop());    int num2 = Integer.valueOf(stack.pop());    int num3 = compute(num1, num2, token);    ...}And it results in following error:Input:["4","3","-"]Output:-1Expected:Fix:if (token.equals("+") || token.equals("-") || token.equals("*") || token.equals("/")) {    int num1 = Integer.valueOf(stack.pop());    int num2 = Integer.valueOf(stack.pop());    int num3 = compute(num2, num1, token);    ...}

Solution:

public class Solution {    public int evalRPN(String[] tokens) {        if (tokens == null)            throw new IllegalArgumentException("The passed in tokens array in null");        if (tokens.length == 0)            return 0;        Stack<Integer> stack = new Stack<Integer> ();        for (String token : tokens) {            if (token.equals("+") || token.equals("-") || token.equals("*") || token.equals("/")) {                int num1 = Integer.valueOf(stack.pop());                int num2 = Integer.valueOf(stack.pop());                int num3 = compute(num2, num1, token);                stack.push(num3);            } else{                int num = Integer.valueOf(token);                stack.push(num);            }        }        return stack.pop();    }        private int compute(int num1, int num2, String token) {        int res = 0;        switch (token) {            case "+" :                res = num1 + num2;                break;            case "-" :                res = num1 - num2;                break;            case "*" :                res = num1 * num2;                break;            case "/" :                res = num1 / num2;                break;        }        return res;    }}

 

[LeetCode#150]Evaluate Reverse Polish Notation