Dijkstra 双栈算数表达式求值法 

输入输出采用了 算法4 java库

package first_java;

import java.util.Stack;

import edu.princeton.cs.algs4.StdIn;
import edu.princeton.cs.algs4.StdOut;

public class Evaluate {
	public static void main(String[] args) {
		Stack<Double> vals = new Stack<Double>();
		Stack<String> ops = new Stack<String>();
		while(!StdIn.isEmpty()) {
			String str = StdIn.readString();
			if(str.equals("("));
			else if(str.equals("+"))
				ops.push(str);
			else if(str.equals("-"))
				ops.push(str);
			else if(str.equals("*"))
				ops.push(str);
			else if(str.equals("/"))
				ops.push(str);
			else if(str.equals("sqrt"))
				ops.push(str);
			else if(str.equals(")")) {
				double v = vals.pop();
				String op = ops.pop();
				if(op.equals("+")) {
					v = vals.pop() + v;
					vals.push(v);
				}
				else if(op.equals("-")) {
					v = vals.pop() - v;
					vals.push(v);
				}
				else if(op.equals("*")) {
					v = vals.pop() * v;
					vals.push(v);
				}
				else if(op.equals("/")) {
					v = vals.pop() / v;
					vals.push(v);
				}
				else if(op.equals("sqrt")) {
					v = Math.sqrt(v);
					vals.push(v);
				}
			}
			else vals.push(Double.parseDouble(str));

		}
		StdOut.println("The final anwser is :  " + vals.pop());
	}

}