Solved using recursion

NeetCodeRoadmap/Stack/150_Evaluate_Reverse_Polish_Notation.py

@@ -5,52 +5,40 @@ class Solution(object):
         :type tokens: List[str]
         :rtype: int
         """
-        valid_tokens = {
-            '+': lambda x, y: x + y,
-            '-': lambda x, y: x - y,
-            '*': lambda x, y: x * y,
-            '/': lambda x, y: x / y,
-        }
-        tmp = []
+        # operators = {
+        #     '+': lambda x, y: x + y,
+        #     '-': lambda x, y: x - y,
+        #     '*': lambda x, y: x * y,
+        #     '/': lambda x, y: x / y,
+        # }
+        # stack = [] # Space: O(n) 
+        # for token in tokens: # Time: O(n)
+        #     if token not in operators:
+        #         stack.append(int(token))
+        #     else:
+        #         x = stack.pop(-2)
+        #         y = stack.pop()
+        #         stack.append(math.ceil(operators[token](x, y)))
+        # return int(stack[-1])
 
-        # while len(tokens) != 1:
-        blanks = 0
-        count = 0
-        for x in range(len(tokens)):
-            # print(tokens[x])
-            if tokens[x] in valid_tokens:
-                operator = valid_tokens[tokens[x]]
-                if len(tmp) > 1 and count == 0:
-                    a = tmp.pop()
-                    b = tmp.pop()
-                    tokens.append(math.floor(operator(b, a)))
-                    # print(tmp)
-                    # print(tokens)
-                    tokens[x] = None
-                    count += 1
-                else:
-                    a = tmp.pop()
-                    b = tokens.pop()
-                    tokens.append(math.ceil(operator(a, b)))
-                    tokens[x] = None
-            else:
-                tmp.append(int(tokens[x]))
-                tokens[x] = None
-            # print(tokens)
-            # print(tmp)
-        for y in tokens:
-            if y is None:
-                blanks += 1
-                # print(blanks)
-        tokens = tokens[blanks:]
-        if tokens[0] < 0:
-            tokens[0] *= -1
-        return int(tokens[0])
+        # Recursion Solution
+        s = tokens[len(tokens) - 1]
+        tokens.pop()
+        print(tokens)
+        if(s not in '+-/*'): return int(s)
+        else:
+            a = self.evalRPN(tokens)
+            b = self.evalRPN(tokens)
+            if(s == "+"): return a + b
+            elif(s == "*"): return a * b
+            elif(s == "-"): return b - a
+            else: return int(float(b)/a)
 
 obj = Solution()
-# print(obj.evalRPN(["2","1","+","3","*"])) # 9
-# print(obj.evalRPN(["4","13","5","/","+"])) # 6
-# print(obj.evalRPN(["10","6","9","3","+","-11","*","/","*","17","+","5","+"])) # 22
-# print(obj.evalRPN(["0","3","/"])) # 0
+print(obj.evalRPN(["2","1","+","3","*"])) # 9
+print(obj.evalRPN(["4","13","5","/","+"])) # 6
+print(obj.evalRPN(["10","6","9","3","+","-11","*","/","*","17","+","5","+"])) # 22
+print(obj.evalRPN(["0","3","/"])) # 0
 print(obj.evalRPN(["3","11","+","5","-"])) # 9
-print(obj.evalRPN(["3","-4","+"])) # -1
+print(obj.evalRPN(["3","-4","+"])) # -1
+print(obj.evalRPN(["3","11","5","+","-"])) # -13