算法Sedgewick第四版-第1章基础-007一用两个栈实现简单的编译器

1.

 package algorithms.util;

 import algorithms.ADT.Stack;

 /******************************************************************************
  *  Compilation:  javac Evaluate.java
  *  Execution:    java Evaluate
  *  Dependencies: Stack.java
  *
  *  Evaluates (fully parenthesized) arithmetic expressions using
  *  Dijkstra's two-stack algorithm.
  *
  *  % java Evaluate
  *  ( 1 + ( ( 2 + 3 ) * ( 4 * 5 ) ) )
  *  101.0
  *
  *  % java Evaulate
  *  ( ( 1 + sqrt ( 5 ) ) / 2.0 )
  *  1.618033988749895
  *
  *
  *  Note: the operators, operands, and parentheses must be
  *  separated by whitespace. Also, each operation must
  *  be enclosed in parentheses. For example, you must write
  *  ( 1 + ( 2 + 3 ) ) instead of ( 1 + 2 + 3 ).
  *  See EvaluateDeluxe.java for a fancier version.
  *
  *
  *  Remarkably, Dijkstra's algorithm computes the same
  *  answer if we put each operator *after* its two operands
  *  instead of *between* them.
  *
  *  % java Evaluate
  *  ( 1 ( ( 2 3 + ) ( 4 5 * ) * ) + )
  *  101.0
  *
  *  Moreover, in such expressions, all parentheses are redundant!
  *  Removing them yields an expression known as a postfix expression.
  *  1 2 3 + 4 5 * * +
  *
  *
  ******************************************************************************/

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

         while (!StdIn.isEmpty()) {
             String s = StdIn.readString();
             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(")")) {
                 String op = ops.pop();
                 double v = vals.pop();
                 if      (op.equals("+"))    v = vals.pop() + v;
                 else if (op.equals("-"))    v = vals.pop() - v;
                 else if (op.equals("*"))    v = vals.pop() * v;
                 else if (op.equals("/"))    v = vals.pop() / v;
                 else if (op.equals("sqrt")) v = Math.sqrt(v);
                 vals.push(v);
             }
             else vals.push(Double.parseDouble(s));
         }
         StdOut.println(vals.pop());
     }
 }