diff --git a/Python/house-robber.py b/Python/house-robber.py
index db2379811..3efd6e8a0 100644
--- a/Python/house-robber.py
+++ b/Python/house-robber.py
@@ -26,5 +26,16 @@ def rob(self, num):
         
         return num_i
 
+    def rob2(self, nums):
+        """
+        :type nums: List[int]
+        :rtype: int
+        """
+        last, now = 0, 0
+        for i in nums:
+            last, now = now, max(last + i, now)
+        return now
+
+
 if __name__ == '__main__':
         print Solution().rob([8,4,8,5,9,6,5,4,4,10])
diff --git a/Python/number-complement.py b/Python/number-complement.py
new file mode 100644
index 000000000..0adb83481
--- /dev/null
+++ b/Python/number-complement.py
@@ -0,0 +1,32 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+# Given a positive integer, output its complement number. The complement strategy is to flip the bits of its binary representation.
+#
+# Note:
+# The given integer is guaranteed to fit within the range of a 32-bit signed integer.
+# You could assume no leading zero bit in the integer’s binary representation.
+# Example 1:
+# Input: 5
+# Output: 2
+# Explanation: The binary representation of 5 is 101 (no leading zero bits), and its complement is 010. So you need to output 2.
+# Example 2:
+# Input: 1
+# Output: 0
+# Explanation: The binary representation of 1 is 1 (no leading zero bits), and its complement is 0. So you need to output 0.
+
+
+class Solution(object):
+    def findComplement(self, num):
+        """
+        :type num: int
+        :rtype: int
+        """
+        return 2 ** (len(bin(num)) - 2) - 1 - num
+
+
+class Solution2(object):
+    def findComplement(self, num):
+        i = 1
+        while i <= num:
+            i <<= 1
+        return (i - 1) ^ num
diff --git a/Python/number-of-segments-in-a-string.py b/Python/number-of-segments-in-a-string.py
index 066b98208..1a5c0086f 100644
--- a/Python/number-of-segments-in-a-string.py
+++ b/Python/number-of-segments-in-a-string.py
@@ -13,6 +13,7 @@
 # Input: "Hello, my name is John"
 # Output: 5
 
+
 class Solution(object):
     def countSegments(self, s):
         """
@@ -24,3 +25,10 @@ def countSegments(self, s):
             if s[i] == ' ' and s[i-1] != ' ':
                 result += 1
         return result
+
+    def countSegments2(self, s):
+        """
+        :type s: str
+        :rtype: int
+        """
+        return len([i for i in s.strip().split(' ') if i])
diff --git a/Python/power-of-four.py b/Python/power-of-four.py
index b8da27b99..74ca8217a 100644
--- a/Python/power-of-four.py
+++ b/Python/power-of-four.py
@@ -29,3 +29,13 @@ def isPowerOfFour(self, num):
         while num and not (num & 0b11):
             num >>= 2
         return (num == 1)
+
+
+class Solution3(object):
+    def isPowerOfFour(self, num):
+        """
+        :type num: int
+        :rtype: bool
+        """
+        num = bin(num)
+        return True if num[2:].startswith('1') and len(num[2:]) == num.count('0') and num.count('0') % 2 and '-' not in num else False
diff --git a/Python/ransom-note.py b/Python/ransom-note.py
index 347cc0548..917334555 100644
--- a/Python/ransom-note.py
+++ b/Python/ransom-note.py
@@ -41,7 +41,7 @@ def canConstruct(self, ransomNote, magazine):
 
 # Time:  O(n)
 # Space: O(1)
-from collections import Counter
+import collections
 
 class Solution2(object):
     def canConstruct(self, ransomNote, magazine):
@@ -50,4 +50,4 @@ def canConstruct(self, ransomNote, magazine):
         :type magazine: str
         :rtype: bool
         """
-        return not Counter(ransomNote) - Counter(magazine)
+        return not collections.Counter(ransomNote) - collections.Counter(magazine)