add postfix solns

lintcode/271-prefix-notation-to-postfix-notation/python3/solution.py

@@ -0,0 +1,49 @@
+from typing import (
+    List,
+)
+
+from collections import deque
+
+class Solution:
+    """
+    @param expression: A string array
+    @return: The Reverse Polish notation of this expression
+    """
+    def convert_to_r_p_n(self, expression: List[str]) -> List[str]:
+        op_priority = {
+            '/': 1 << 4,
+            '*': 1 << 4,
+            '+': 1 << 2,
+            '-': 1 << 2,
+            '(': 1 << 0
+        }
+
+        op_stack = deque()
+        result = []
+
+        for token in expression:
+            if token.isdigit():
+                result.append(token)
+
+            elif token == '(':
+                op_stack.append(token)
+
+            elif token == ')':
+                while len(op_stack) > 0 and op_stack[-1] != '(':
+                    result.append(op_stack.pop())
+
+                op_stack.pop()
+
+            else:
+                # For any other operator, keep popping and appending to results as long as there
+                # are higher priority operators in the stack
+                while len(op_stack) > 0 and op_priority[op_stack[-1]] >= op_priority[token]:
+                    result.append(op_stack.pop())
+                
+                op_stack.append(token)
+        
+        # Pop and append the remaining operators in the stack
+        while len(op_stack) > 0:
+            result.append(op_stack.pop())
+        
+        return result