[LeetCode系列]Evaluate Reverse Polish Notation

Evaluate 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



思考：观察例子，可以发现，操作符的处理优先级是根据数组的前后顺序作为优先级进行处理，所以没有必要对操作符进行入栈操作。
而对当前的操作符进行计算时，会取栈顶位置的头两个数进行运算，运算完将结果压入栈，此题不难理解，直接上代码：


代码：

public class Solution {
    public int evalRPN(String[] tokens) {
        Stack<Integer> stack = new Stack<Integer>();
        String operators = "+-*/";
        for (String s:tokens) {
            if (!operators.contains(s)) {
                int num = Integer.valueOf(s);
                stack.push(num);
            } else {
                int a = 0; 
                int b = 0;
                if (!stack.isEmpty()) {
                     a = Integer.valueOf(stack.pop());
                }
                if (!stack.isEmpty()) {
                     b = Integer.valueOf(stack.pop());
                }
                int index = operators.indexOf(s);
                int c = 0;
                switch (index) {
                    case 0:
                           c = a + b;
                           stack.push(c);
                           break;
                    case 1:
                           c = b - a;
                           stack.push(c);
                           break;
                    case 2:
                           c = a * b;
                           stack.push(c);
                           break;
                    case 3:
                           c = b / a;
                           stack.push(c);
                           break;
                }
            }
        }
        int value = stack.pop();
        return value;
    }
}