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Postfix to Prefix conversion in Java:
As we have already discussed about infix ,prefix, and postfix operation .
In this java tutorial we will discuss how to convert an postfix to prefix operation in java.
for example :
- Input : Postfix : AB+CD-*
- Output : Prefix : *+AB-CD
Let us see how to this this operation :


ALGORITHM:
Move to the given expression from left to right, one at a time
- If the character is operand do push operation to stack.
- If the character is operator then do the following
- Pop operand from the stack, say it’s p1.
- Pop operand from the stack, say it’s p2.
- do operation on p1 and p2 and push in stack.
- Once the expression end initialize the result string and pop out from the stack and add it to the result.
- show the result.
Java code :
import java.util.Stack;
public class PostFixToPreFix {
static boolean isOperator(char x){
switch (x){
case ‘-‘:
case ‘+’:
case ‘/’:
case ‘*’:
case ‘^’:
return true;
}
return false;
}
public static String convert(String exp){
Stack<String> st = new Stack<>();
for (int i = 0; i exp.length() ; i++) {
char c = exp.charAt(i);
if(isOperator(c)){
String s1 = st.pop();
String s2 = st.pop();
String temp = c + s2 + s1;
st.push(temp);
}else{
st.push(c+“”);
}
}
String result = st.pop();
return result;
}
public static void main(String[] args) {
String postfix = “AB+CD-*”;
System.out.println(“Postfix exp: “ + postfix);
System.out.println(“Prefix exp: “ + convert(postfix));
}
}
output:
postfix exp:
AB+CD-*
prefix exp:
*+AB-CD
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