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.

 1 class Solution {
 2     static boolean isOperator(char x) {
 3         switch (x) {
 4             case '+':
 5             case '-':
 6             case '/':
 7             case '*':
 8                 return true;
 9             default:
10                 return false;
11         }
12     }
13 
14     static String postToPre(String exp) {
15         Stack<String> s = new Stack<>();
16         int length = exp.length();
17         for (int i = 0; i < length; i++) {
18             if (isOperator(exp.charAt(i))) {
19                 String op1 = s.pop();
20                 String op2 = s.pop();
21                 String temp = exp.charAt(i) + op2 + op1;
22                 s.push(temp);
23             }
24             else {
25                 s.push(exp.charAt(i) + "");
26             }
27         }
28         return s.peek();
29     }
30 }

Prefix to Postfix Conversion

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.

 1 class Solution {
 2     boolean isOperator(char x) {
 3         switch (x) {
 4             case '+':
 5             case '-':
 6             case '/':
 7             case '*':
 8                 return true;
 9             default:
10                 return false;
11         }
12     }
13 
14     String preToPost(String pre_exp) {
15         Stack<String> s = new Stack<>();
16         int length = pre_exp.length();
17         for (int i = length - 1; i >= 0; i--) {
18             if (isOperator(pre_exp.charAt(i))) {
19                 String op1 = s.pop();
20                 String op2 = s.pop();
21                 String temp = op1 + op2 + pre_exp.charAt(i);
22                 s.push(temp);
23             } else {
24                 s.push(pre_exp.charAt(i) + "");
25             }
26         }
27         return s.peek();
28     }
29 }

Postfix to Prefix Conversion & Prefix to Postfix Conversion

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