add postfix solns

lintcode/271-prefix-notation-to-postfix-notation/README.md

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+Given a string array representing an expression, and return the Reverse Polish notation of this expression. (remove the parentheses)
+
+Definition of _Reverse Polish Notation_:
+
+* [https://en.wikipedia.org/wiki/Reverse\_Polish\_notation](https://en.wikipedia.org/wiki/Reverse_Polish_notation)
+
+Example
+
+Example 1:
+    
+        Input: ["3", "-", "4", "+", "5"] Output: ["3", "4", "-", "5", "+"] Explanation: 3 - 4 + 5 = -1 + 5 = 4 3 4 - 5 + = -1 5 + = 4
+    
+    
+
+Example 2:
+    
+        Input: ["(", "5", "-", "6", ")", "*", "7"] Output: ["5","6","-","7","*"] Explanation: (5 - 6) * 7 = -1 * 7 = -7 5 6 - 7 * = -1 7 * = -7

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

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+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