Leetcode

1. 多行注释：ctrl + /

2. enumerae()函数

将一个可遍历的数据对象组合为一个索引序列，同时列出数据和数据下标，如：

seq = ['one', 'two', 'three']
for i, element in enumerate(seq):
　　print(i, element)

0 one

1 two

3 three

3. find shortest string in list, key=len可忽略

min(list_name, key = len)

4. 取余数：%，整除（向下取整）：//

5. Tree

Binary Search Tree作用：speed up our binaryy search as we are able to find items faster

Tree traversal: traversing means visiting and outputting the value of each node in a particular order

*3种Depth First Traversals*

a) In-order: In the case of binary search trees (BST), Inorder traversal gives nodes in non-decreasing order

Until all nodes are traversed-
Step 1: recursively traverse left subtree
Step 2: visit root node
Step 3: recursively traverse right subtree

b) Pre-order: Preorder traversal is used to create a copy of the tree. Preorder traversal is also used to get prefix expression on an expression tree

Until all nodes are traversed-
Step 1: visit root node
Step 2: recursively traverse left subtree
Step 3: recursively traverse right subtree

c) Post-order: Postorder traversal is used to delete the tree or get the postfix expression of an expression tree

Until all nodes are traversed-
Step 1:recursively traverse left subtree
Step 2: recursively traverse right subtree
Step 3: visit root node

Depth First Traversals: 
(a) Inorder (Left, Root, Right) : 4 2 5 1 3 
(b) Preorder (Root, Left, Right) : 1 2 4 5 3 
(c) Postorder (Left, Right, Root) : 4 5 2 3 1