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406. Evaluation of Postfix Expressions (Polish Postfix notation)  Set 2
Earlier we had discussed how to evaluate postfix expressions where operands are of singledigit. In this article, we will discuss how to evaluate postfix expressions for any number ( not necessarily single digit.)
Postfix notation is a notation for writing arithmetic expressions in which the operands appear before 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 above postfix is: 500 + 40 which resolves to 540 Postfix: 20 50 3 6 + * * 300 / 2  Output: 28 Explanation: Infix expression of above postfix is: 20 * (50 *(3+6))/3002 which resolves to 28
Approach: Use Stack
Algorithm:
Iterate through given expression, one character at a time
 If the character is a digit, initialize number = 0
 while the next character is digit
 do number = number*10 + currentDigit
 push number to the stack.
 while the next character is digit
 If the character is an operator,
 pop operand from the stack, say it's s1.
 pop operand from the stack, say it's s2.
 perform (s2 operator s1) 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:
Postfix Expression: 20 50 3 6 + * * 300 / 2  Evaluation: 28.0