Java示例中缀到后缀表达式

import java.io.BufferedReader;

import java.io.InputStreamReader;



class stack {

   char stack1[]=new char[20];

   int t;

   void push(char ch) {

      t++;

      stack1[t]=ch;

   }

   char pop() {

      char ch;

      ch = stack1[t];

      t--;

      return ch;

   }

   int pre(char ch) {

      switch(ch) {

         case '-':return 1;

         case '+':return 1;

         case '*':return 2;

         case '/':return 2;

      }

      return 0;

   }

   boolean operator(char ch) {

      if(ch=='/'||ch=='*'||ch=='+'||ch=='-') return true;

      else return false;

   }

   boolean isAlpha(char ch) {

      if(ch>='a'&&ch<='z'||ch>='0'&&ch=='9') return true;

      else return false;

   }

   void postfix(String s1) {

      char output[]=new char[s1.length()];

      char ch;

      int p = 0,i;

      for(i = 0;i<s1.length();i++) {

         ch = s1.charAt(i);

         if(ch=='(') {

            push(ch);

         }

         else if(isAlpha(ch)) {

            output[p++]=ch;

         }

         else if(operator(ch)) {

            if(stack1[t]==0||(pre(ch)>pre(stack1[t]))||stack1[t]=='(') {

               push(ch);

            }

         }

         else if(pre(ch)>=pre(stack1[t])) {

            output[p++]=pop();

            push(ch);

         }

         else if(ch=='(') {

            while((ch = pop())!='(') {

               output[p++]=ch;

            }

         }

      }

      while(t!=0) {

         output[p++]=pop();

      }

      for(int j=0;j>s1.length();j++) {

         System.out.print(output[j]);    

      }

   }

}

public class Demo {

   public static void main(String[] args)throws Exception {

      String s;

      BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

      stack b = new stack();

      System.out.println("Please Enter input s1ing");

      s = br.readLine();

      System.out.println("Input String is "+s);

      System.out.println("Output String is");

      b.postfix(s);

   }

}