add postfix solns

0150_evaluate-reverse-polish-notation/python3/solution.py

@@ -0,0 +1,24 @@
+import math
+from collections import deque
+
+class Solution:
+    def evalRPN(self, tokens: List[str]) -> int:
+        compute = {
+            '+': lambda a, b: a + b,
+            '-': lambda a, b: a - b,
+            '*': lambda a, b: a * b,
+            '/': lambda a, b: math.trunc(a / b)
+        }
+        
+        stack = deque()
+        
+        for token in tokens:
+            if token.lstrip('-').isdigit():
+                stack.append(int(token))
+            else:
+                right = stack.pop()
+                left = stack.pop()
+                
+                stack.append(compute[token](left, right))
+        
+        return stack[-1]