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Author: Chris Wu

problems/binary-tree-inorder-traversal.py

@@ -0,0 +1,18 @@
+"""
+In-order traversal means that: if there is a left node, visit it first; visit the current node; then if there is right node, visit it.
+In-order: Left->Current->Right
+Pre-order: Current->Left->Right
+Post-order: Left->Right->Current
+"""
+class Solution(object):
+    def inorderTraversal(self, root):
+        def helper(node):
+            if not node: return
+            helper(node.left)
+            opt.append(node.val)
+            helper(node.right)
+            
+        opt = []
+        helper(root)
+        return opt
+        

problems/design-linked-list.py

@@ -5,21 +5,21 @@ def __init__(self, val):
 
 class MyLinkedList(object):
     def __init__(self):
-        self.l = []
+        self.l = [] #trade space for speed
 
-    def get(self, index):
+    def get(self, index): #O(1)
         if index<0 or index>=len(self.l):
             return -1
         return self.l[index].val
 
-    def addAtHead(self, val):
+    def addAtHead(self, val): #O(N), list extend takes O(N), <https://wiki.python.org/moin/TimeComplexity>.
         self.l = [Node(val)]+self.l
 
-    def addAtTail(self, val):
+    def addAtTail(self, val): #O(N)
         self.l = self.l+[Node(val)]
 
-    def addAtIndex(self, index, val):
+    def addAtIndex(self, index, val): #O(N)
         self.l = self.l[:index]+[Node(val)]+self.l[index:]
 
-    def deleteAtIndex(self, index):
+    def deleteAtIndex(self, index): #O(N)
         self.l = self.l[:index]+self.l[index+1:]