算术表达式求值(中缀转后缀,后缀求值，java 栈实现)

package com.operator;import java.util.Arrays;import java.util.LinkedList;public class OperatorTest{//定义一个枚举，来保持操作符enum OperartorEnum{additive("+"),//加号subtraction("-"),//减号multiplication("*"),//乘号division("/"),//除号leftbracket("("),//左括号rightbracket(")");//右括号private  String va;public String getVal(){return va;}OperartorEnum(String s){va=s;}}public static void main(String[] args){String s="3.8/2+222/(23-21)-45*2";//定义一个字符串表达式OperatorTest test=new OperatorTest();String[] inArr=test.ConvertStringToArr(s);System.out.println("中缀表达式:"+s+"="+(3.8/2+222/(23-21)-45*2));test.showArr(inArr);String[] poArr=test.ConvertIntoPofix(inArr);System.out.println("后缀表达式:");test.showArr(poArr);double result=test.CalculatePoix(poArr);System.out.println("后缀表达式的计算结果="+result);}//计算后缀表达式的值//基本思想，依次扫描后缀表达式，遇到数字则入栈；//遇到操作符，将栈顶的两个数字弹出，计算值，再入栈public double CalculatePoix(String[] poArr){double result=0;double op1=0;double op2=0;LinkedList<String> list=new LinkedList<String>();//构造一个运算符栈for(String ch:poArr){//如果遇到'+','-','*','/'，则计算if(ch.equals(OperartorEnum.additive.getVal())||ch.equals(OperartorEnum.subtraction.getVal())||ch.equals(OperartorEnum.multiplication.getVal())||ch.equals(OperartorEnum.division.getVal())){op2=Double.valueOf(list.pop());// 先弹出的是第二个操作数，并删除op1=Double.valueOf(list.pop());// 先弹出的是第二个操作数,并删除list.push(String.valueOf(CalculateExpression(op1,op2,ch)));//计算值，再入栈}else{list.push(ch);//否则为数字，直接进栈}}//循环后，栈只剩下一个数字，即为最终的表达式的值result=Double.valueOf(list.pop());return result;}//计算一个最简单的二元运算表达式public double CalculateExpression(double op1,double op2,String op){double result=0;if(op.equals(OperartorEnum.additive.getVal())){result=op1+op2;}else if(op.equals(OperartorEnum.subtraction.getVal())){result=op1-op2;}else if(op.equals(OperartorEnum.multiplication.getVal())){result=op1*op2;}else if(op.equals(OperartorEnum.division.getVal())){result=op1/op2;}return result;}//将中缀表达式转化为后缀表达式public String[] ConvertIntoPofix(String[] inArr){LinkedList<String> list=new LinkedList<String>();int index=0;String poArr[]=new String[13];for(String s:inArr){if(s!=null){//如果是加号或者减号,将栈中操作符弹出，直到"("if(s.equals(OperartorEnum.additive.getVal())||s.equals(OperartorEnum.subtraction.getVal())){while(!list.isEmpty()){String temp=list.getFirst();if(!temp.equals(OperartorEnum.leftbracket.getVal())){poArr[index++]=list.pop();}else{break;}}list.push(s);}//如果为乘号或者除号，则弹出栈顶的除号和乘号，直到"("else if(s.equals(OperartorEnum.multiplication.getVal())||s.equals(OperartorEnum.division.getVal())){while(!list.isEmpty()){String temp=list.getFirst();//如果为乘号，或者除号，并且不等于"("if((temp.equals(OperartorEnum.multiplication.getVal())||temp.equals(OperartorEnum.division.getVal()))&&(!temp.equals(OperartorEnum.leftbracket.getVal()))){poArr[index++]=list.pop();}else{break;}}list.push(s);}//遇到"(",直接进栈else if(s.equals(OperartorEnum.leftbracket.getVal())){list.push(s);}//如果遇到右括号，则将栈顶的操作符弹出，直到"(",再删除"("else if(s.equals(OperartorEnum.rightbracket.getVal())){while(!list.isEmpty()){String temp=list.getFirst();if(!temp.equals(OperartorEnum.leftbracket.getVal())){poArr[index++]=list.pop();}else{break;}}list.pop();}else{poArr[index++]=s;}//System.out.println("后缀为:"+Arrays.toString(poArr)+",栈为:"+list );}}while(!list.isEmpty()){poArr[index++]=list.pop();//System.out.println("后缀为:"+Arrays.toString(poArr)+",栈为:"+list );}return poArr;}//将一个字符串表达式转换为一个数组public  String[] ConvertStringToArr(String s){String arr[]=new String[s.length()];int index=0;for(int i=0;i<s.length();){String ch=String.valueOf(s.charAt(i));if(ch.equals(OperartorEnum.additive.getVal())||ch.equals(OperartorEnum.subtraction.getVal())||ch.equals(OperartorEnum.multiplication.getVal())||ch.equals(OperartorEnum.division.getVal())||ch.equals(OperartorEnum.leftbracket.getVal())||ch.equals(OperartorEnum.rightbracket.getVal())){arr[index++]=ch;i++;}else{int end=SearchEnd(s,i);arr[index++]=s.substring(i, end);i=end;}}return arr;}//查找一个数的结束索引(这样做的原因是一个数可能是多位的，并且带有小数点)private  int SearchEnd(String s,int index){int result=index;int i=0;for(i=index;i<s.length();i++){String ch=String.valueOf(s.charAt(i));if(ch.equals(OperartorEnum.additive.getVal())||ch.equals(OperartorEnum.subtraction.getVal())||ch.equals(OperartorEnum.multiplication.getVal())||ch.equals(OperartorEnum.division.getVal())||ch.equals(OperartorEnum.leftbracket.getVal())||ch.equals(OperartorEnum.rightbracket.getVal())){result=i;break;}}if(i==s.length()){result=s.length();}return result;}//打印一个数组private void showArr(String[] arr){for(String ss: arr){System.out.println(ss!=null?ss:"");}}}

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