Added postfix expression evaluation program in python

python/postfixexpression_evaluation.py

@@ -0,0 +1,34 @@
+'''
+Evaluate the value of an arithmetic expression in Reverse Polish Notation.
+Valid operators are +, -, *, /.Each operand may be an integer or another expression.
+
+'''
+
+class Solution:
+    def evaluateRPN(self, A):
+        stack=[]
+        top=-1
+        for i in A:
+            if(i!='+' and i!='-' and i!='*' and i!='/'):
+                stack.append(i)
+                top+=1
+                continue
+            else:
+                y=int(stack.pop())
+                top-=1
+                x=int(stack.pop())
+                top-=1
+                if(i=='+'):
+                    stack.append(x+y)
+                    top+=1
+                elif(i=='-'):
+                    stack.append(x-y)
+                    top+=1
+                elif(i=='*'):
+                    stack.append(x*y)
+                    top+=1
+                elif(i=='/'):
+                    stack.append(x//y)
+                    top+=1
+        return stack[top]
+