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

/*************************************************************************
*  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());
}
}

/*************************************************************************
*  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());
}
}