# JAVA数据结构示例---逆波兰式求值!（转）

package expression;
public class Calculate{
public static boolean isOperator(String operator){
if(operator.equals("+")||operator.equals("-")||operator.equals("*")||operator.equals("/")||operator.equals("(")||operator.equals(")")) return true;
else return false;

package expression;
public class Calculate{
public static boolean isOperator(String operator){
if(operator.equals("+")||operator.equals("-")||operator.equals("*")||operator.equals("/")||operator.equals("(")||operator.equals(")")) return true;
else return false;
}
public static int priority(String operator){
if(operator.equals("+")||operator.equals("-")||operator.equals("(")) return 1;
else if(operator.equals("*")||operator.equals("/")) return 2;
else return 0;
}
public static String twoResult(String operator,String a,String b){
try{
String op=operator;
String rs=new String();
double x=Double.parseDouble(b);
double y=Double.parseDouble(a);
double z=0;
if(op.equals("+")) z=x+y;
else if(op.equals("-")) z=x-y;
else if(op.equals("*")) z=x*y;
else if(op.equals("/")) z=x/y;
else z=0;
return rs+z;
}
catch(NumberFormatException e){
System.out.println("input has something wrong!");
return "Error";
}
}
}

package expression;
import java.util.*;
public class Stacks{
int top=-1;
public void push(Object value){
top++;
}
public Object pop(){
Object temp=list.getFirst();
top--;
list.removeFirst();
return temp;

}
public Object top(){
return list.getFirst();
}
}

package expression;
import java.io.*;
import java.util.*;
public class Expression{
private ArrayList expression=new ArrayList();//存储中序表达式
private ArrayList right=new ArrayList();//存储右序表达式
private String result;//结果
//依据输入信息创建对象，将数值与操作符放入ArrayList中
private Expression(String input){
StringTokenizer st=new StringTokenizer(input,"+-*/()",true);
while(st.hasMoreElements()){
}
}
//将中序表达式转换为右序表达式
private void toRight(){
Stacks aStack=new Stacks();
String operator;
int position=0;
while(true){
if(Calculate.isOperator((String)expression.get(position))){
if(aStack.top==-1||((String)expression.get(position)).equals("(")){
aStack.push(expression.get(position));
}
else{
if(((String)expression.get(position)).equals(")")){
if(!((String)aStack.top()).equals("(")){
operator=(String)aStack.pop();
}
}
else{
if(Calculate.priority((String)expression.get(position))<=Calculate.priority((String)aStack.top())&&aStack.top!=-1){
operator=(String)aStack.pop();
}
aStack.push(expression.get(position));
}
}
}
position++;
if(position>=expression.size()) break;
}
while(aStack.top!=-1){
operator=(String)aStack.pop();
}
}
//对右序表达式进行求值
private void getResult(){
this.toRight();
Stacks aStack=new Stacks();
String op1,op2,is=null;
Iterator it=right.iterator();
while(it.hasNext()){
is=(String)it.next();
if(Calculate.isOperator(is)){
op1=(String)aStack.pop();
op2=(String)aStack.pop();
aStack.push(Calculate.twoResult(is,op1,op2));
}
else aStack.push(is);
}
result=(String)aStack.pop();
it=expression.iterator();
while(it.hasNext()){
System.out.print((String)it.next());
}
System.out.println("="+result);
}
public static void main(String avg[]){
try{
System.out.println("Input a expression:");
for(;;){
String input=new String();
if(input.equals("q")) break;
else{
Expression boya=new Expression(input);
boya.getResult();
}
System.out.println("Input another expression or input ´q´ to quit:");
}
is.close();
}
catch(IOException e){
System.out.println("Wrong input!!!");
}
}
}

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