LeetCode150 Evaluate Reverse Polish Notation java题解

题目：

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：

这题是栈的一个经典应用，也比较简单，思路就是：遇到数字就进栈，遇到运算符号就出栈两个数字然后再将计算结果进栈。

代码：

import java.util.Stack;

public class LeetCode150_EvaluateReversePolishNotation {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		String[] s={"4","-2","/","2","-3","-","-"};
		System.out.println(evalRPN(s));

	}

	 public static int evalRPN(String[] tokens) {
		 int length=tokens.length;
		 Stack<Integer> stack =new Stack<>();

		 for(int i=0;i<length;i++)
		 {
			 if(!isOperator(tokens[i]))
				 stack.push(Integer.parseInt(tokens[i]));
			 else {
				int operaNum2=stack.pop();
				int operaNum1=stack.pop();
				if(tokens[i].equals("+"))
					stack.push(operaNum1+operaNum2);
				else if(tokens[i].equals("-"))
					stack.push(operaNum1-operaNum2);
				else if(tokens[i].equals("*"))
					stack.push(operaNum1*operaNum2);
				else {
					stack.push(operaNum1/operaNum2);
				}
			}

		 }

		 return stack.pop();

	    }

	 public static boolean isOperator(String s)
	 {
		 if(s.equals("+")||s.equals("-")||s.equals("*")||s.equals("/"))
			 return true;
		 else
			 return false;
	 }

}

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