Evaluate Reverse Polish Notation @LeetCode

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

	package leetcode;
	import java.util.Stack;
	/**
	* Solution: Use stack store numbers and calculate when meets operator.(get number from stack and store result into stack)
	* Use string.contains(substr) to check operator
	* Don't forget break!!!
	* @author jeffwan
	* @date Apr 1, 2014
	*/
	public class EvalRPN {
	public int evalRPN(String[] tokens) {
	if (tokens == null || tokens.length == 0) {
	return 0;
	}
	Stack<Integer> stack = new Stack<Integer>();
	String operators = "+-*/";
	for (String token : tokens) {
	if (!operators.contains(token)) {
	stack.push(Integer.parseInt(token));
	continue;
	}
	int a = stack.pop();
	int b = stack.pop();
	switch(token.charAt(0)) {
	case '+':
	stack.push(b + a);
	break;
	case '-':
	stack.push(b - a);
	break;
	case '*':
	stack.push(b * a);
	break;
	case '/':
	stack.push(b / a);
	break;
	}
	}
	return stack.pop();
	}
	// My ways is not very clear
	public int evalRPN2(String[] tokens) {
	Stack<String> stack = new Stack<String>();
	for (int i = 0; i < tokens.length; i++) {
	System.out.println(stack);
	if (isNumberic(tokens[i])) {
	stack.push(tokens[i]);
	} else {
	int number1 = Integer.parseInt(stack.pop());
	int number2 = Integer.parseInt(stack.pop());
	// deal with the operator
	int result = calculate(number2, number1, tokens[i]);
	stack.push(Integer.toString(result));
	}
	}
	return Integer.parseInt(stack.peek());
	}
	public int calculate(int number1, int number2, String operator) {
	switch (operator.charAt(0)) {
	case '+':
	return number1 + number2;
	case '-':
	return number1 - number2;
	case '*':
	return number1 * number2;
	case '/':
	return number1 / number2;
	default:
	return 0;
	}
	}
	public boolean isNumberic(String number) {
	return number.matches("-?\\d+(\\.\\d+)?");
	}
	}

view raw EvalRPN.java hosted with ❤ by GitHub