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
解题思路：
计算逆波兰表达式，Stack的经典应用，JAVA实现如下：
public int evalRPN(String[] tokens) {
if (tokens == null || tokens.length == 0)
return 0;
Stack<Integer> stack = new Stack<Integer>();
for (String str : tokens) {
if (str.length() > 1 || Character.isDigit(str.charAt(0)))
stack.push(Integer.parseInt(str));
else {
int a = stack.pop();
int b = stack.pop();
if (str.equals("+"))
stack.push(b + a);
else if (str.equals("-"))
stack.push(b - a);
else if (str.equals("*"))
stack.push(b * a);
else if (str.equals("/"))
stack.push(b / a);
}
}
return stack.pop();
}