算法Sedgewick第四版-第1章基础-014一用stack把前置表达式转为后置表达式并计算值

1.

 /*************************************************************************
  *  Exercise 1.3.10
  *
  *  % java InfixToPostfix
  *  ( 1 + ( ( 2 + 3 ) * ( 4 * 5 ) ) )
  *  1 2 3 + 4 5 * * +
  *
  *  % java InfixToPostfix
  *  ( sqrt ( 1 + 2 ) )
  *  1 2 + sqrt
  *
  *************************************************************************/

 public class InfixToPostfix
 {
     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("("))               ;
             else if (s.equals("+") ||
                      s.equals("-") ||
                      s.equals("*") ||
                      s.equals("/") ||
                      s.equals("sqrt")) ops.push(s);
             else if (s.equals(")"))
             {
                 String op = ops.pop();
                 String v = vals.pop();

                 if (op.equals("+") ||
                     op.equals("-") ||
                     op.equals("*") ||
                     op.equals("/"))
                     v = String.format("%s %s %s", vals.pop(), v, op);
                 else if (op.equals("sqrt"))
                     v = String.format("%s %s", v, op);

                 vals.push(v);
             }
             else vals.push(s);
         }

         StdOut.println(vals.pop());
     }
 }

2.

 /*************************************************************************
  *  Exercise 1.3.11
  *
  *  % java EvaluatePostfix
  *  1 2 3 + 4 5 * * +
  *  101.0
  *
  *  % java EvaluatePostfix
  *  1 5 sqrt + 2.0 /
  *  1.618033988749895
  *
  *  % java EvaluatePostfix
  *  12 9 - 105 7 / *
  *  45.0
  *
  *  % java InfixToPostfix | java EvaluatePostfix
  *  ( 1 + ( ( 2 + 3 ) * ( 4 * 5 ) ) )
  *  101.0
  *
  *  % java InfixToPostfix | java EvaluatePostfix
  *  ( ( 1 + sqrt ( 5 ) ) / 2.0 )
  *  1.618033988749895
  *
  *************************************************************************/

 public class EvaluatePostfix
 {
     public static void main(String[] args)
     {
         Stack<Double> vals = new Stack<Double>();

         while (!StdIn.isEmpty())
         {
             String s = StdIn.readString();

             if      (s.equals("(") ||
                      s.equals(")")) ;
             else if (s.equals("+") ||
                      s.equals("-") ||
                      s.equals("*") ||
                      s.equals("/") ||
                      s.equals("sqrt"))
             {
                 double v = vals.pop();

                 if      (s.equals("+"))    v = vals.pop() + v;
                 else if (s.equals("-"))    v = vals.pop() - v;
                 else if (s.equals("*"))    v = vals.pop() * v;
                 else if (s.equals("/"))    v = vals.pop() / v;
                 else if (s.equals("sqrt")) v = Math.sqrt(v);

                 vals.push(v);
             }
             else
                 vals.push(Double.parseDouble(s));
         }

         StdOut.println(vals.pop());
     }
 }