227 Basic Calculator II

Author: idf

227 Basic Calculator II py3.py

@@ -0,0 +1,89 @@
+#!/usr/bin/python3
+"""
+Implement a basic calculator to evaluate a simple expression string.
+
+The expression string contains only non-negative integers, +, -, *, / operators
+and empty spaces . The integer division should truncate toward zero.
+
+Example 1:
+
+Input: "3+2*2"
+Output: 7
+Example 2:
+
+Input: " 3/2 "
+Output: 1
+Example 3:
+
+Input: " 3+5 / 2 "
+Output: 5
+Note:
+
+You may assume that the given expression is always valid.
+Do not use the eval built-in library function.
+"""
+
+
+class Solution:
+    def calculate(self, s: str) -> int:
+        """
+        No brackets. Look at previous operand and operator, when finishing
+        scanning current operand.
+        """
+        operand = 0
+        stk = []
+        prev_op = "+"
+        for i, c in enumerate(s):
+            if c.isdigit():
+                operand = operand * 10 + int(c)
+
+            #  i == len(s) - 1
+            delimited = c in ("+", "-", "*", "/") or i == len(s) - 1
+            if delimited:
+                if prev_op == "+":
+                    cur = operand
+                elif prev_op == "-":
+                    cur = -operand
+                elif prev_op == "*":
+                    cur = stk.pop() * operand
+                else:
+                    assert prev_op == "/"
+                    # instead of op1 // op2 due to negative handling, -3 // 2 == -2
+                    cur = int(stk.pop() / operand)
+
+                stk.append(cur)
+                prev_op = c
+                operand = 0
+
+        return sum(stk)
+
+    def calculate_error(self, s: str) -> int:
+        """
+        cannot use dictionary, since it is eager evaluation
+        """
+        operand = 0
+        stk = []
+        prev_op = "+"
+        for i, c in enumerate(s):
+            if c.isdigit():
+                operand = operand * 10 + int(c)
+
+            #  i == len(s) - 1
+            delimited = c in ("+", "-", "*", "/") or i == len(s) - 1
+            if delimited:
+                cur = {
+                    "+": operand,
+                    "-": -operand,
+                    "*": stk.pop() * operand,
+                    "/": int(stk.pop() / operand),  # instead of op1 // op2 due to negative handling, -3 // 2 == -2
+                }[prev_op]
+                stk.append(cur)
+
+                prev_op = c
+                operand = 0
+
+        return sum(stk)
+
+
+if __name__ == "__main__":
+    assert Solution().calculate("3+2*2") == 7

227 Basic Calculator II.py

@@ -23,11 +23,14 @@ def calculate(self, s):
         :type s: str
         :rtype: int
         """
-        lst = self.to_list(s)
+        lst = self.parse(s)
         post = self.infix2postfix(lst)
         return self.eval_postfix(post)
 
-    def to_list(self, s):
+    def parse(self, s):
+        """
+        return tokens
+        """
         i = 0
         ret = []
         while i < len(s):
@@ -47,7 +50,8 @@ def to_list(self, s):
         return ret
 
     def infix2postfix(self, lst):
-        stk = []  # store operators in strictly increasing precedence
+        # operator stacks rather than operand
+        stk = []  # stk only stores operators in strictly increasing precedence
         ret = []
         for elt in lst:
             if elt.isdigit():
@@ -100,4 +104,4 @@ def eval_postfix(self, post):
 
 
 if __name__ == "__main__":
-    assert Solution().calculate("3+2*2") == 7
+    assert Solution().calculate("3+2*2") == 7