150. 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

链接： http://leetcode.com/problems/evaluate-reverse-polish-notation/

6/13/2017

15ms, 42%

用stack来保存之前的值，注意最后一步返回stack的剩余元素

public class Solution {
    public int evalRPN(String[] tokens) {
        if (tokens == null) {
            return 0;
        }
        Stack<Integer> stack = new Stack<Integer>();
        int result = 0;
        for (int i = 0; i < tokens.length; i++) {
            if (tokens[i].equals("+") || tokens[i].equals("-") || tokens[i].equals("*") || tokens[i].equals("/")) {
                int operand2 = stack.pop();
                int operand1 = stack.pop();
                if (tokens[i].equals("+")) {
                    result = operand1 + operand2;
                } else if (tokens[i].equals("-")) {
                    result = operand1 - operand2;
                } else if (tokens[i].equals("*")) {
                    result = operand1 * operand2;
                } else {
                    result = operand1 / operand2;
                }
                stack.push(result);
            } else {
                stack.push(Integer.parseInt((tokens[i])));
            }
        }
        return stack.pop();
    }
}

更多讨论

https://discuss.leetcode.com/category/158/evaluate-reverse-polish-notation