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416. Evaluation of Prefix Expressions (Polish Notation)  Set 2
Earlier we had discussed how to evaluate prefix expression where operands are of singledigit. In this article, we will discuss how to evaluate prefix expression for any number ( not necessarily single digit.)
Prefix notation is a notation for writing arithmetic expressions in which the operands appear after their operators. Let's assume the below
 Operands are real numbers (could be multiple digits).
 Permitted operators: +,, *, /, ^(exponentiation)
 Blanks are used as a separator in expression.
 Parenthesis are permitted
Example:
Postfix: + 500 40 Output: 540 Explanation: Infix expression of the above prefix is: 500 + 40 which resolves to 540 Postfix:  / * 20 * 50 + 3 6 300 2 Output: 28 Explanation: Infix expression of above prefix is: 20 * (50 *(3+6))/3002 which resolves to 28
Approach: Use Stack
Algorithm:
Reverse the given expression and Iterate through it, one character at a time
 If the character is a digit, initialize String temp;
 while the next character is not a digit
 do temp = temp + currentDigit
 convert Reverse temp into Number.
 push Number to the stack.
 while the next character is not a digit
 If the character is an operator,
 pop the operand from the stack, say it's s1.
 pop the operand from the stack, say it's s2.
 perform (s1 operator s2) and push it to stack.
 Once the expression iteration is completed, The stack will have the final result. Pop from the stack and return the result.
Please see the walkthrough of an example below for more understanding.
Output:
Prefix Expression:  / * 20 * 50 + 3 6 300 2 Evaluation: 28.0