Question

construct an expression tree, then print the preorder of the tree for each expression

Write this in Java, test all cases below

7 * ( 3 - 6 ) + 5 => + * 7 - 3 6 5 13.0 +

18 / -4 => + 13.0 / 18 -4

0.11 / 5 * ( 0.2 * 7 ) => * / 0.11 5 * 0.2 7

-0.27 * 2.21 + 3 - -0.7 / -2

2.05 - 5 + 3 -3 - 1 + 2 + -3.4 / 5

( 855 / 11 ) * ( 12.2 / 321 - ( 1 + 2 + ( 1 + 1.1 + ( 12.51 - 0.51 ) ) ) ) - 0.11

-0.12 + 1.22 - -12.1 / 2.1 => - + -0.12 1.22 / -12.1 2.1

0.5 * 8 / ( 12 + 5 * ( 2 + 3 / 2 ) ) - ( 5 + 1 ) * 3 / 2.5 ==> - / * 0.5 8 + 12 * 5 + 2 / 3 2 / * + 5 1 3 2.5

( -5 + 2 ) / -3 * 2 - -1 / 3 * ( ( 5 / -2) + 1.5 ) => - * / + -5 2 -3 2 * / -1 3 + / 5 -2 1.5

Answer 1

package javaapplication3;
import java.io.*;
class Stack
{
private char[] a;
private int top,m;
public Stack(int max)
{
m=max;
a=new char[m];
top=-1;
}
public void push(char key)
{
a[++top]=key;
}
public char pop()
{
return(a[top--]);
}
public char peek()
{
return(a[top]);
}
public boolean isEmpty()
{
return (top==-1);
}
}
class preoder
{
private Stack s;
private String input;
private String output="";
public preoder(String str)
{
input=str;
s=new Stack(str.length());
}
public String pre()
{
for(int i=input.length()-1;i>=0;i--)
{
char ch=input.charAt(i);
switch(ch)
{
case '+':
case '-':gotOperator(ch,1,')');
break;
case '*':
case '/':gotOperator(ch,2,')');
break;
case ')':s.push(ch);
break;
case '(':gotParenthesis(')');
break;
default:output=ch+output;
}
}
while(!s.isEmpty())
output=s.pop()+output;
return output;
}

private void gotOperator(char opThis,int prec1,char x)
{
while(!s.isEmpty())
{
char opTop=s.pop();
if(opTop==x)
{
s.push(opTop);
break;
}
else
{
int prec2;
if(opTop=='+'||opTop=='-')
prec2=1;
else
prec2=2;
if(prec2<prec1&&x=='(')
{
s.push(opTop);
break;
}
else if(prec2<=prec1&&x==')')
{
s.push(opTop);
break;
}
else
{
if(x==')')
output=opTop+output;
else
output=output+opTop;
}
}
}
s.push(opThis);
}
private void gotParenthesis(char x)
{
while(!s.isEmpty())
{
char ch=s.pop();
if(ch==x)
break;
else
{
if(x==')')
output=ch+output;
else
output=output+ch;
}
}
}
}
class conversion
{
public static void main(String args[])throws IOException
{
String s;
preoder exp;
DataInputStream inp=new DataInputStream(System.in);
  
System.out.println("Enter the infix expression ");
s=inp.readLine();
exp=new preoder(s);
System.out.println("Prefix expression:- "+exp.pre());   
  
}
}

Note: Put proper parenthesis while providing the input