Dijkstra's Two-stack Algorithm for Expression Evaluation(Java)

tags: stack Algorithms java Expression evaluation

Use stack to calculate the input formula (there is a space between each operator and number)

import java.util.Scanner;
import java.util.LinkedList;
import java.util.Queue;

import edu.princeton.cs.algs4.Stack;

public class Evaluate {
	public static void main(String[] args){
		Stack<String> ops=new Stack<String>();
		Stack<Double> values=new Stack<Double>();
		Scanner sc=new Scanner(System.in);
		String next=sc.nextLine();
		String[] expression=next.split("\\s+");
		Queue<String> que=new LinkedList<String>();
		for (String x : expression)
			que.add(x);
		while(!que.isEmpty()){
			//push if operator
			String s=que.poll();
			if(s.equals("("))	;
			else if(s.equals("+"))	ops.push(s);
			else if(s.equals("-"))	ops.push(s);
			else if(s.equals("*"))	ops.push(s);
			else if(s.equals("/"))	ops.push(s);
			else if(s.equals("sqrt"))	ops.push(s);
			else if(s.equals(")")){
			//pop,evaluate and push result if token is")"
				String op=ops.pop();
				double v=values.pop();
				if(op.equals("+"))	v=values.pop()+v;
				else if(op.equals("-"))v=values.pop()-v;
				else if(op.equals("*"))v=values.pop()*v;
				else if(op.equals("/"))v=values.pop()/v;
				else if(op.equals("sqrt"))v=Math.sqrt(v);					
				values.push(v);
			}
			else values.push(Double.parseDouble(s));
		}
		System.out.print(values.pop());
	}
}

input: ( 1 + ( ( 2 + 3 ) * ( 4 * 5 ) ) )

output: 101.0

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