Update Readme.

binary-tree-maximum-path-sum.py

Author: Chris Wu

README.md

@@ -8,29 +8,32 @@ https://github.com/wuduhren/leetcode-python
 
 # Resources
 These are the interview resources I personally used and only if it is really helpful I will put it here.
-## System Design
-More resource  
-<https://github.com/shashank88/system_design>  
-Architecture 101  
-<https://engineering.videoblocks.com/web-architecture-101-a3224e126947>  
-How to scale up? There are also lots of tech interview related topic in his channel.  
-<https://www.youtube.com/watch?v=yPF94QiI2qk&t=385s>
 
-## Data Structure and Algorithm Basics
-Basic data structure and algorithm online course taught in Python. This course is design to help you find job and do well in the interview.  
-<https://classroom.udacity.com/courses/ud513>
+## Overall Mindset
+Haveing a right mindset is the most important resource. It keeps you going when you are tired after work. Studying when everyone else are out having fun. Reminding you that your goals are not going to come easy, it takes time, self-discipline, mental and physical toughness...
+
+This is a speech from Sean Lee on "How to Get a Job at the Big 4". I highly recommend this because it gives me the right mindset on how to put in the work.  
+<https://youtu.be/YJZCUhxNCv8>
+
+CS DoJo on "How I Got a Job at Google as a Software Engineer". There are also lots of technique on coding interview in his channel.  
+<https://www.youtube.com/watch?v=UPO-9iMjBpc>
 
 ## Prepare in a Structural Way
 1. <https://www.quora.com/How-should-I-prepare-for-my-Google-interview-if-I-have-1-month-left-and-I%E2%80%99m-applying-for-a-software-engineer-role/answer/Anthony-D-Mays?ch=10&share=5c488000&srid=W0jqp>
 2. <https://www.quora.com/How-can-I-get-a-job-at-Facebook-or-Google-in-6-months-I-need-a-concise-work-plan-to-build-a-good-enough-skill-set-Should-I-join-some-other-start-up-or-build-my-own-projects-start-up-Should-I-just-focus-on-practicing-data-structures-and-algorithms/answer/Jimmy-Saade>
 3. <https://www.quora.com/What-should-I-know-from-the-CLRS-3rd-edition-book-if-my-aim-is-to-get-into-Google/answer/Jimmy-Saade>
 
-## Overall Mindset
-This is a speech from Sean Lee on "How to Get a Job at the Big 4". I highly recommend this because it gives me the right mindset on how to put in the work.  
-<https://youtu.be/YJZCUhxNCv8>
+## Data Structure and Algorithm Basics
+Basic data structure and algorithm online course taught in Python. This course is design to help you find job and do well in the interview.  
+<https://classroom.udacity.com/courses/ud513>
 
-CS DoJo on "How I Got a Job at Google as a Software Engineer". There are also lots of technique on coding interview in his channel.  
-<https://www.youtube.com/watch?v=UPO-9iMjBpc>
+## System Design
+More resource  
+<https://github.com/shashank88/system_design>  
+Architecture 101  
+<https://engineering.videoblocks.com/web-architecture-101-a3224e126947>  
+How to scale up? There are also lots of tech interview related topic in his channel.  
+<https://www.youtube.com/watch?v=yPF94QiI2qk&t=385s>
 
 ## Interview Question Survey
 <https://www.glassdoor.com/index.htm>  

binary-tree-maximum-path-sum.py

@@ -0,0 +1,67 @@
+"""
+For every subtree, we generate two output.
+
+* The output from the path which can be used by its parent.
+    * Only the node itself
+    * The node with its avaliable left branch
+    * The node with its avaliable right branch
+
+* The output from the path which cannot (or won't) be used by its parent.
+    * Only the path from the left branch
+    * Only the path from the right branch
+    * The node with avaliable left branch and avaliable right branch at the same time
+    * The output of the first case. The means that even the path can be selected but we do not select it. Because selecting it will generate smaller output.
+
+We initial all the value to be negative infinity, so the it will just be ignored by the `max()` if left or right node is `None`.
+`a` means avaliable.
+`na` means not avaliable.
+"""
+#recursive
+class Solution(object):
+    def maxPathSum(self, root):
+        def helper(node):
+            left_a = left_na = right_a = right_na = float('-inf')
+
+            if node.left:
+                left_a, left_na = helper(node.left)
+            if node.right:
+                right_a, right_na = helper(node.right)
+
+            a = max(node.val, node.val+left_a, node.val+right_a)
+            na = max(left_na, right_na, node.val+left_a+right_a, a)
+            return a, na
+
+        return max(helper(root))
+
+#iterative
+class Solution(object):
+    def maxPathSum(self, root):
+        pre_stack = [root]
+        stack = []
+        memo = {}
+
+        while pre_stack:
+            node = pre_stack.pop()
+            stack.append(node)
+            if node.left:
+                pre_stack.append(node.left)
+            if node.right:
+                pre_stack.append(node.right)
+
+        while stack:
+            node = stack.pop()
+
+            left_a = left_na = right_a = right_na = float('-inf')
+
+            if node.left:
+                left_a, left_na = memo[node.left]
+            if node.right:
+                right_a, right_na = memo[node.right]
+
+            a = max(node.val, node.val+left_a, node.val+right_a)
+            na = max(left_na, right_na, node.val+left_a+right_a, a)
+
+            memo[node] = (a, na)
+
+        return max(memo[root])
+