150. Evaluate Reverse Polish Notation

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

用stack解题。遍历字符串数组，如果是符号，从stack里弹出两个数进行对应的计算，如果是数字，转化成对应值之后入栈，最后结果返回stack的仅存的一个数。代码如下：

public class Solution {
    public int evalRPN(String[] tokens) {
        if(tokens.length == 0) {
            return 0;
        }
        Stack<Integer> stack = new Stack<Integer>();
        int num1 = 0, num2 = 0;
        for (String str: tokens) {
            if (str.equals("+")) {
                num2 = stack.pop();
                num1 = stack.pop();
                stack.push(num1 + num2);
            } else if (str.equals("-")) {
                num2 = stack.pop();
                num1 = stack.pop();
                stack.push(num1 - num2);
            } else if (str.equals("*")) {
                num2 = stack.pop();
                num1 = stack.pop();
                stack.push(num1 * num2);
            } else if (str.equals("/")) {
                num2 = stack.pop();
                num1 = stack.pop();
                stack.push(num1 / num2);
            } else {
                stack.push(Integer.parseInt(str));
            }
        }
        return stack.pop();
    }
}