add postfix solns

0150_evaluate-reverse-polish-notation/README.md

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+Evaluate the value of an arithmetic expression in [Reverse Polish Notation](http://en.wikipedia.org/wiki/Reverse_Polish_notation).
+
+Valid operators are `+`, `-`, `*`, and `/`. Each operand may be an integer or another expression.
+
+**Note** that division between two integers should truncate toward zero.
+
+It is guaranteed that the given RPN expression is always valid. That means the expression would always evaluate to a result, and there will not be any division by zero operation.
+
+**Example 1:**
+
+    Input: tokens = ["2","1","+","3","*"]
+    Output: 9
+    Explanation: ((2 + 1) * 3) = 9
+    
+
+**Example 2:**
+
+    Input: tokens = ["4","13","5","/","+"]
+    Output: 6
+    Explanation: (4 + (13 / 5)) = 6
+    
+
+**Example 3:**
+
+    Input: tokens = ["10","6","9","3","+","-11","*","/","*","17","+","5","+"]
+    Output: 22
+    Explanation: ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
+    = ((10 * (6 / (12 * -11))) + 17) + 5
+    = ((10 * (6 / -132)) + 17) + 5
+    = ((10 * 0) + 17) + 5
+    = (0 + 17) + 5
+    = 17 + 5
+    = 22
+    
+
+**Constraints:**
+
+*   `1 <= tokens.length <= 104`
+*   `tokens[i]` is either an operator: `"+"`, `"-"`, `"*"`, or `"/"`, or an integer in the range `[-200, 200]`.

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

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+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]