算法Sedgewick第四版-第1章基础-020一按优先级计算表达式的值

 /******************************************************************************
  *  Compilation:  javac EvaluateDeluxe.java
  *  Execution:    java EvaluateDeluxe
  *  Dependencies: Stack.java
  *
  *  Evaluates arithmetic expressions using Dijkstra's two-stack algorithm.
  *  Handles the following binary operators: +, -, *, / and parentheses.
  *
  *  % echo "3 + 5 * 6 - 7 * ( 8 + 5 )" | java EvaluateDeluxe
  *  -58.0
  *
  *
  *  Limitiations
  *  --------------
  *    -  can easily add additional operators and precedence orders, but they
  *       must be left associative (exponentiation is right associative)
  *    -  assumes whitespace between operators (including parentheses)
  *
  *  Remarks
  *  --------------
  *    -  can eliminate second phase if we enclose input expression
  *       in parentheses (and, then, could also remove the test
  *       for whether the operator stack is empty in the inner while loop)
  *    -  see http://introcs.cs.princeton.edu/java/11precedence/ for
  *       operator precedence in Java
  *
  ******************************************************************************/
 
 import java.util.TreeMap;
 
 public class EvaluateDeluxe {
 
     // result of applying binary operator op to two operands val1 and val2
     public static double eval(String op, double val1, double val2) {
         if (op.equals("+")) return val1 + val2;
         if (op.equals("-")) return val1 - val2;
         if (op.equals("/")) return val1 / val2;
         if (op.equals("*")) return val1 * val2;
         throw new RuntimeException("Invalid operator");
     }
 
     public static void main(String[] args) {
 
         // precedence order of operators
         TreeMap<String, Integer> precedence = new TreeMap<String, Integer>();
         precedence.put("(", 0);   // for convenience with algorithm
         precedence.put(")", 0);
         precedence.put("+", 1);   // + and - have lower precedence than * and /
         precedence.put("-", 1);
         precedence.put("*", 2);
         precedence.put("/", 2);
 
         Stack<String> ops  = new Stack<String>();
         Stack<Double> vals = new Stack<Double>();
 
         while (!StdIn.isEmpty()) {
 
             // read in next token (operator or value)
             String s = StdIn.readString();
 
             // token is a value
             if (!precedence.containsKey(s)) {
                 vals.push(Double.parseDouble(s));
                 continue;
             }
 
             // token is an operator
             while (true) {
 
                 // the last condition ensures that the operator with higher precedence is evaluated first
                 if (ops.isEmpty() || s.equals("(") || (precedence.get(s) > precedence.get(ops.peek()))) {
                     ops.push(s);
                     break;
                 }
 
                 // evaluate expression
                 String op = ops.pop();
 
                 // but ignore left parentheses
                 if (op.equals("(")) {
                     assert s.equals(")");
                     break;
                 }
 
                 // evaluate operator and two operands and push result onto value stack
                 else {
                     double val2 = vals.pop();
                     double val1 = vals.pop();
                     vals.push(eval(op, val1, val2));
                 }
             }
         }
 
         // finished parsing string - evaluate operator and operands remaining on two stacks
         while (!ops.isEmpty()) {
             String op = ops.pop();
             double val2 = vals.pop();
             double val1 = vals.pop();
             vals.push(eval(op, val1, val2));
         }
 
         // last value on stack is value of expression
         StdOut.println(vals.pop());
         assert vals.isEmpty();
         assert ops.isEmpty();
     }
 }