Prefix to Infix Conversion

Infix : An expression is called the Infix expression if the operator appears in between the operands in the expression. Simply of the form (operand1 operator operand2).
Example : (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 Prefix expression, convert it into a Infix 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 between them.
• string = (operand1 + operator + operand2)
• 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 preToInfix(String pre_exp) {
         Stack<String> stack = new Stack<>();
         int length = pre_exp.length();
         for (int i = length - ; i >= ; i--) {
             if (isOperator(pre_exp.charAt(i))) {
                 String op1 = stack.pop();
                 String op2 = stack.pop();

                 String temp = "(" + op1 + pre_exp.charAt(i) + op2 + ")";
                 stack.push(temp);
             } else {
                 stack.push(pre_exp.charAt(i) + "");
             }
         }
         return stack.peek();
     }
 }