用stack计算逆波兰表达式

Stack的原理：

public class Node {
	int val;
	Node next;

	Node(int val) {
		this.val = val;
	}
}

Stack 实现机制：

public class Stack {
	Node top;

	public Node peek() {
		if (top != null) {
			return top;
		}
		return null;
	}

	public Node pop() {
		if (top == null)
			return null;
		else {
			Node tmp = new Node(top.val);
			top = top.next;
			return tmp;
		}
	}

	public void push(Node n) {
		if (n != null) {
			n.next = top;
			top = n;
		}
	}
}

逆波兰表达式例子：
["1", "2", "+", "3", "*"] -> ((1 + 2) * 3) -> 9
["4", "10", "5", "/", "+"] -> (4 + (10 / 5)) -> 6

我们可以直接用JDK提供的Stack来计算逆波兰表达式的值：

public class Test {

	public static void main(String[] args) throws IOException {
		String[] tokens = new String[] { "1", "2", "+", "3", "*" };
		System.out.println(compute(tokens));
	}

	public static int compute(String[] tokens) {
		int returnValue = 0;
		String operators = "+-*/";

		Stack<String> stack = new Stack<String>();

		for (String t : tokens) {
			if (!operators.contains(t)) {
				stack.push(t);
			} else {
				int a = Integer.valueOf(stack.pop());
				int b = Integer.valueOf(stack.pop());
				switch (t) {
				case "+":
					stack.push(String.valueOf(a + b));
					break;
				case "-":
					stack.push(String.valueOf(b - a));
					break;
				case "*":
					stack.push(String.valueOf(a * b));
					break;
				case "/":
					stack.push(String.valueOf(b / a));
					break;
				}
			}
		}

		returnValue = Integer.valueOf(stack.pop());

		return returnValue;
	}
}