Evaluate Reverse Polish Notation/ Reverse Words in a String

Author: gavinfish

150 Evaluate Reverse Polish Notation.py

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+'''
+Evaluate the value of an arithmetic expression in Reverse Polish Notation.
+
+Valid operators are +, -, *, /. Each operand may be an integer or another expression.
+
+Some examples:
+  ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
+  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6
+'''
+
+class Solution(object):
+    def evalRPN(self, tokens):
+        """
+        :type tokens: List[str]
+        :rtype: int
+        """
+        stack = []
+        for token in tokens:
+            if token not in ("+", "-", "*", "/"):
+                stack.append(int(token))
+            else:
+                second = stack.pop()
+                first = stack.pop()
+                if token == "+":
+                    stack.append(first + second)
+                elif token == "-":
+                    stack.append(first - second)
+                elif token == '*':
+                    stack.append(first * second)
+                else:
+                    if first * second < 0:
+                        stack.append(-(abs(first) // abs(second)))
+                    else:
+                        stack.append(first // second)
+        return stack.pop()
+
+
+if __name__ == "__main__":
+    assert Solution().evalRPN(["2", "1", "+", "3", "*"]) == 9
+    assert Solution().evalRPN(["4", "13", "5", "/", "+"]) == 6
+    assert Solution().evalRPN(["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]) == 22

151 Reverse Words in a String.py

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+'''
+Given an input string, reverse the string word by word.
+
+For example,
+Given s = "the sky is blue",
+return "blue is sky the".
+
+Update (2015-02-12):
+For C programmers: Try to solve it in-place in O(1) space.
+
+Clarification:
+What constitutes a word?
+A sequence of non-space characters constitutes a word.
+Could the input string contain leading or trailing spaces?
+Yes. However, your reversed string should not contain leading or trailing spaces.
+How about multiple spaces between two words?
+Reduce them to a single space in the reversed string.
+'''
+
+class Solution(object):
+    def reverseWords(self, s):
+        """
+        :type s: str
+        :rtype: str
+        """
+        return " ".join(s.split()[::-1])
+
+
+if __name__ == "__main__":
+    assert Solution().reverseWords("the sky is blue ") == "blue is sky the"