# Problem

Given an integer, return its base 7 string representation.

Example 1:

Input: 100
Output: "202"


Example 2:

Input: -7
Output: "-10"


Note:Ā The input will be in range of [-1e7, 1e7].

# Solution: Simulation

Time complexity: O(logn)

Space complexity: O(logn)

# Problem

In the following, every capital letter represents some hexadecimal digit fromĀ 0Ā toĀ f.

The red-green-blue colorĀ "#AABBCC"Ā can be writtenĀ asĀ "#ABC"Ā inĀ shorthand.Ā  For example,Ā "#15c"Ā is shorthand for the colorĀ "#1155cc".

Now, say the similarity between two colorsĀ "#ABCDEF"Ā andĀ "#UVWXYZ"Ā isĀ -(AB - UV)^2 -Ā (CD - WX)^2 -Ā (EF - YZ)^2.

Given the colorĀ "#ABCDEF", return a 7 character colorĀ that is most similar toĀ #ABCDEF, and has a shorthand (that is, it can be represented as someĀ "#XYZ"

Example 1:
Input: color = "#09f166"
Output: "#11ee66"
Explanation:
The similarity is -(0x09 - 0x11)^2 -(0xf1 - 0xee)^2 - (0x66 - 0x66)^2 = -64 -9 -0 = -73.
This is the highest among any shorthand color.


## Note:

• colorĀ is a string of lengthĀ 7.
• colorĀ is a valid RGB color: forĀ i > 0,Ā color[i]Ā is a hexadecimal digit fromĀ 0Ā toĀ f
• Any answer which has the same (highest)Ā similarity as the best answer will be accepted.
• All inputs and outputs should use lowercase letters, and the output is 7 characters.

# Solution: Brute Force

R, G, B are independent, find the closest color for each channel separately.

Time complexity: O(3 * 16)

Space complexity: O(1)

Problem:

Serialization is the process of converting a data structure or object into a sequence of bits so that it can be stored in a file or memory buffer, or transmitted across a network connection link to be reconstructed later in the same or another computer environment.

Design an algorithm to serialize and deserialize a binary tree. There is no restriction on how your serialization/deserialization algorithm should work. You just need to ensure that a binary tree can be serialized to a string and this string can be deserialized to the original tree structure.

For example, you may serialize the following tree

asĀ "[1,2,3,null,null,4,5]", just the same asĀ how LeetCode OJ serializes a binary tree. You do not necessarily need to follow this format, so please be creative and come up with different approaches yourself.

Note:Ā Do not use class member/global/static variables to store states. Your serialize and deserialize algorithms should be stateless.

https://leetcode.com/problems/serialize-and-deserialize-binary-tree/description/

Idea:

Recursion

Time Complexity O(n)