也许是周围的人都太优秀了，给自己一种紧张的压迫感。总想着追赶别人，所以总是催促自己，以至于做事情总想快快地做，快快地做。2020 年让自己一步一步地来，一点一点地来。
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LeetCode 430. Flatten a Multilevel Doubly Linked List
You are given a doubly linked list which in addition to the next and previous pointers, it could have a child pointer, which may or may not point to a separate doubly linked list. These child lists may have one or more children of their own, and so on, to produce a multilevel data structure, as shown in the example below.
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LeetCode 429. N-ary Tree Level Order Traversal
Given an n-ary tree, return the level order traversal of its nodes’ values.
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LeetCode 427. Construct Quad Tree
We want to use quad trees to store an N x N boolean grid. Each cell in the grid can only be true or false. The root node represents the whole grid. For each node, it will be subdivided into four children nodes until the values in the region it represents are all the same.
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LeetCode 424. Longest Repeating Character Replacement
Given a string s that consists of only uppercase English letters, you can perform at most k operations on that string.
In one operation, you can choose any character of the string and change it to any other uppercase English character.
Find the length of the longest sub-string containing all repeating letters you can get after performing the above operations.
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LeetCode 423. Reconstruct Original Digits from English
Given a non-empty string containing an out-of-order English representation of digits 0-9, output the digits in ascending order.
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LeetCode 421. Maximum XOR of Two Numbers in an Array
Given a non-empty array of numbers, a0, a1, a2, … , an-1, where 0 ≤ ai < 231. Find the maximum result of ai XOR aj, where 0 ≤ i, j < n. Could you do this in O(n) runtime?
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Python Advance Techs
Some advance tips about python.
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LeetCode 419. Battleships in a Board
Given an 2D board, count how many battleships are in it. The battleships are represented with ‘X’s, empty slots are represented with ‘.’s. You may assume the following rules:
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