算法（Algorithms）第4版 练习 1.3.9

//1.3.9
//only support +-*/ sqrt operator
package com.qiusongde;

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

public class AddLeftParentheses {

    public static void main(String[] args) {
        
        Stack<String> ops = new Stack<String>();
        Stack<String> vals = new Stack<String>();
        
        while(!StdIn.isEmpty()) {
            String s = StdIn.readString();
            
            if(s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/") || s.equals("sqrt")) {
                ops.push(s);
            }
            else if(s.equals(")")) {
                String op = ops.pop();//operator
                String v = vals.pop();//value
                
                if(op.equals("+") || op.equals("-") || op.equals("*") || op.equals("/")) {
                    String subexpression = "( " + vals.pop() + " " + op  + " " + v + " )";
                    vals.push(subexpression);
                }
                
                if(op.equals("sqrt")) {
                    String subexpression = op + " ( " + v + " )";
                    vals.push(subexpression);
                }
                
            }
            else {
                vals.push(s);
            }
            
        }
        
        StdOut.println(vals.pop());
        
    }

}