Postfix to Prefix Conversion & Prefix to Postfix Conversion

Postfix to Prefix Conversion

Postfix: An expression is called the postfix expression if the operator appears in the expression after the operands. Simply of the form (operand1 operand2 operator).
Example : AB+CD-* (Infix : (A+B) * (C-D) )

Prefix : An expression is called the prefix expression if the operator appears in the expression before the operands. Simply of the form (operator operand1 operand2).
Example : *+AB-CD (Infix : (A+B) * (C-D) )

Given a Postfix expression, convert it into a Prefix expression.

分析：

• Read the Postfix expression from left to right
• If the symbol is an operand, then push it onto the Stack
• If the symbol is an operator, then pop two operands from the Stack
• Create a string by concatenating the two operands and the operator before them. Like: string = operator + operand2 + operand1, and push the resultant string back to Stack
• Repeat the above steps until end of Prefix expression.

 class Solution {
     static boolean isOperator(char x) {
         switch (x) {
             case '+':
             case '-':
             case '/':
             case '*':
                 return true;
             default:
                 return false;
         }
     }

     static String postToPre(String exp) {
         Stack<String> s = new Stack<>();
         int length = exp.length();
         for (int i = ; i < length; i++) {
             if (isOperator(exp.charAt(i))) {
                 String op1 = s.pop();
                 String op2 = s.pop();
                 String temp = exp.charAt(i) + op2 + op1;
                 s.push(temp);
             }
             else {
                 s.push(exp.charAt(i) + "");
             }
         }
         return s.peek();
     }
 }

Prefix to Postfix Conversion

Prefix : An expression is called the prefix expression if the operator appears in the expression before the operands. Simply of the form (operator operand1 operand2).
Example : *+AB-CD (Infix : (A+B) * (C-D) )

Postfix: An expression is called the postfix expression if the operator appears in the expression after the operands. Simply of the form (operand1 operand2 operator).
Example : AB+CD-* (Infix : (A+B * (C-D) )

Given a Prefix expression, convert it into a Postfix expression.

分析：

• Read the Prefix expression in reverse order (from right to left)
• If the symbol is an operand, then push it onto the Stack
• If the symbol is an operator, then pop two operands from the Stack
• Create a string by concatenating the two operands and the operator after them.
• string = operand1 + operand2 + operator
• And push the resultant string back to Stack
• Repeat the above steps until end of Prefix expression.

 class Solution {
     boolean isOperator(char x) {
         switch (x) {
             case '+':
             case '-':
             case '/':
             case '*':
                 return true;
             default:
                 return false;
         }
     }

     String preToPost(String pre_exp) {
         Stack<String> s = new Stack<>();
         int length = pre_exp.length();
         for (int i = length - ; i >= ; i--) {
             if (isOperator(pre_exp.charAt(i))) {
                 String op1 = s.pop();
                 String op2 = s.pop();
                 String temp = op1 + op2 + pre_exp.charAt(i);
                 s.push(temp);
             } else {
                 s.push(pre_exp.charAt(i) + "");
             }
         }
         return s.peek();
     }
 }