Evaluate Reverse Polish Notation

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本文介绍了一种使用栈的数据结构来解析并计算逆波兰表达式的算法实现。该方法适用于包含加、减、乘、除操作的有效算术表达式。通过Java代码示例展示了如何逐个处理输入字符串中的元素，并在遇到运算符时执行相应的计算。

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Evaluate Reverse Polish Notation

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

Java代码：

public class Solution {
    public  int evalRPN(String[] tokens) {
		Stack<Integer> stack = new Stack<>();
		if (tokens.length == 0)
			return 0;
		for (int i = 0; i < tokens.length; i++) {
			if (isNum(tokens[i])) {
				stack.push(Integer.parseInt(tokens[i]));
			} else {
				Integer n2 = stack.pop();
				Integer n1 = stack.pop();
				Integer result;
				if (tokens[i].equals("+")) {
					result = n1 + n2;
					stack.push(result);
				} else if (tokens[i].equals("-")) {
					result = n1 - n2;
					stack.push(result);
				} else if (tokens[i].equals("*")) {
					result = n1 * n2;
					stack.push(result);
				} else if (tokens[i].equals("/")) {
					result = n1 / n2;
					stack.push(result);
				}
			}
		}
		return stack.pop();
	}

	public  boolean isNum(String s) {
		try {
			Integer.parseInt(s);
			return true;
		} catch (Exception e) {
			return false;
		}
	}
}