Leekow Learning Plan—Basic Notes on Data Structure and Algorithm

data structure

array

Topic: 136. Numbers that occur only once

Title: 169. Majority elements

Topic: 15. The sum of three numbers (difficult)

This method is what we often call "double pointer". When we need to enumerate two elements in the array, if we find that the second element is decremented as the first element increases, then we can use the double pointer method.

If the number needs to be different from the last enumeration, this technique is also used in permutation and combination problems:
```
if first > 0 and nums[first] == nums[first - 1]:
	continue
```
That is, if they are the same, directly judge the next element

Title: 75. Color classification (Dutch flag problem, classic)

Title: 56. Merging Intervals

Topic: 706. Designing Hash Maps

Title: 119. Yang Hui Triangle II

Title: 48. Rotate an image

Main diagonal flip + horizontal flip, or, y=x flip + vertical flip, the answer is the former, the code is more concise

Title: 59. Spiral Matrix II

Topic: 240. Searching Two-Dimensional Matrices II

Title: 435. Non-overlapping intervals
```
class Solution:
    def eraseOverlapIntervals(self, intervals: List[List[int]]) -> int:
        if not intervals:
            return 0
        
        intervals.sort(key=lambda x: x[1])
        n = len(intervals)
        right = intervals[0][1]
        ans = 1

        for i in range(1, n):
            if intervals[i][0] >= right:
                ans += 1
                right = intervals[i][1]
        
        return n - ans
```
Greedy, sort by the left endpoint of the interval, add the ones that meet the requirements, and delete the rest

Title: 334. Incrementing Ternary Subsequences

Consider left and right; greedy, memorize method

Topic: 238. Product of arrays other than themselves

Prefix sum + suffix sum, IQ rolling

Title: 560. Subarrays whose sum is K
string

Title: 415. Adding Strings

Topic: 409. Longest Palindrome

Topic: 290. Word Rules

Topic: 763. Dividing Alphabet Intervals

Topic: 49. Grouping of anagrams

Title: 43. Multiply Strings

Title: 187. Repeated DNA Sequences

Topic: 5. Longest Palindromic Substring

The horse-drawn cart still doesn't understand, go listen to Zuo Shen again, and write the code yourself
linked list

Topic: 2. Adding two numbers

Title: 142. Circular Linked List II

Topic: 160. Intersecting linked lists (pointing to each other's head nodes)

Title: 82. Delete Duplicate Elements in a Sorted List II

Dumb node + fast and slow pointer implementation does not take up extra space

Topic: 24. Exchange nodes in a linked list pairwise

Topic: 707. Designing Linked Lists

Topic: 25. A group of K flipped linked lists

Topic: 143. Rearranging linked lists (classic questions)
stack/queue

Title: 155. Minimal Stack

It is said that there is a practice of not using additional space (auxiliary stack)

Subject: 1249. Remove Invalid Parentheses

Title: 1823. Find the Winner of the Game ( Joseph Ring )
Tree

Topic: 108. Convert Sorted Array to Binary Search Tree

Title: 105. Construct Binary Trees from Preorder and Inorder Traversals

Title: 103. Zigzag Level Order Traversal of Binary Trees

Title: 199. Right View of a Binary Tree

You can also use dfs

Title: 113. Path sums II

(Unscheduled) Topic: 437. Pathways to Sum III

Use the prefix and

Title: 450. Delete a node in a binary search tree (classic question)

Topic: 230. The Kth Smallest Element in a Binary Search Tree

Title: 173. Binary Search Tree Iterators

Title: 236. The nearest common ancestor of a binary tree

Title: 297. Serialization and deserialization of binary trees
picture

Topic: 997. Finding the Town's Judge

Title: 1557. The Minimum Number of Points All Points Can Be Reached

Topic: 841. Keys and Rooms
priority queue (heap)

Title: 215. The Kth Largest Element in an Array

Title: 347. Top K High Frequency Elements

Topic: 451. Sorting Characters by Frequency

Title: 973. The K closest points to the origin

algorithm

two points

Topic: 34. Find the first and last position of an element in a sorted array

Topic: 33. Searching a Rotated Sorted Array (This question is very troublesome, the answer is very good)

Topic: 74. Searching a two-dimensional matrix (2 degree bisection is more common)

Topic: 153. Find the minimum value in a rotated sorted array

Topic: 162. Finding Peaks (Auxiliary Functions Handling Boundaries, Conditional Simplification)
double pointer

Title: 82. Delete Duplicate Elements in a Sorted List II

Elegant solution, one traversal

Topic: 15. The sum of three numbers

God K's solution is better understood, the second brush still can't (OvO)

Subject: 844. Comparing Strings with Backspaces

Simple questions, but examine the coding ability

Title: 986. Intersection of Interval Lists

Topic: 11. The container that holds the most water
sliding window

Topic: 438. Find all anagrams in a string (emphasis)

This question is very similar to the previous one. You can use diff to record different numbers, so you only need to judge the situation of entry and exit

Title: 713. Subarrays whose product is less than K

Title: 209. Minimum Length Subarray
dfs/bfs

Topic: 200. Number of Islands

Title: 547. Number of Provinces

As an introductory question for dfs/bfs of a graph, it is an undirected and acyclic graph

Title: 117. Filling the next right node pointer of each node II

Topic: 572. Subtree of another tree (this simple question is not easy)

Title: 1091. Shortest Paths in a Binary Matrix
- dfs: determine whether the path/result exists
- bfs: Find the shortest/optimal path
Here is a solution to the problem, the analysis of dfs and bfs is better, and why the visited of bfs can be used globally, https://leetcode-cn.com/problems/shortest-path-in-binary-matrix/solution/bfszui-duan-lu-jing-wen-ti-bfsdfsde-si-k-ngc5/

Topic: 130. Surrounded areas

Reverse Thinking

Title: 797. All Possible Paths

Directed acyclic graph, memorized together with the above
recursion/backtracking

Title: 78. Subsets

Title: 90. Subset II

Subset problem:
1. Backtracking method, increase the condition as the length of the path, add a valid answer after the length is satisfied, and return
2. binary mask
Contains duplicate values: sort the array, de-duplicate processing (the last one is not selected, and the current one is the same as the previous one, skip it)

(Unplanned) Topic: 46. Full Arrangement

Title: 47. Full Arrangement II

Arrangement problem:
1. Backtracking method, the most direct and classic solution, but it is not easy to deal with the situation where there are repeated elements
2. Exchange method, taking K God as an example
With duplicate values:
1. (For the backtracking method) sorting, using dictionaries and element judgments at the same time, the judgment here is more complicated and difficult to understand
2. (for the exchange method) only dictionary processing is required
Backtracking method:

Specifically, we only need to set a rule in the recursive function to ensure that repeated characters will only be filled once when filling each blank. Specifically, we first sort the original string to ensure that the same characters are adjacent to each other. In the recursive function, we limit that the character filled in each time must be "the first unfilled character from left to right" in the repeated character set where this character is located, that is, the following judgment conditions :
```
if j in vis or (j-1 not in vis and j > 0 and nums[j] == nums[j-1]):
	continue
```
Here, the element cannot be hashed directly, only the subscript corresponding to the element can be hashed, and then judged:
```
class Solution:
    def permuteUnique(self, nums: List[int]) -> List[List[int]]:
        def backtrack(i):
            # 满足条件，这里不能用输入变量了
            if len(path) == len(nums):
                res.append(path[:])
                return
            
            # 要考虑所有的元素，而不是只考虑当前位置后面的元素，所以用for
            for j in range(len(nums)):
                # 上一个选择和本次重复
                # 这里为什么是或？

                # j已经选择过 或者 j-1没选过但是j和j-1对应元素相等
                if j in vis or (j-1 not in vis and j > 0 and nums[j] == nums[j-1]):
                    continue
                
                vis.add(j)
                path.append(nums[j])
                backtrack(j)
                path.pop()
                vis.remove(j)
        
        res = []
        path = []
        vis = set()
        nums.sort()
        backtrack(0)
        return res
```
Exchange method:
```
class Solution:
    def permuteUnique(self, nums: List[int]) -> List[List[int]]:
        # 交换法
        def backtrack(i):
            if i == len(nums)-1:
                res.append(nums[:])
                return
            dct = set()
            for j in range(i, len(nums)):
                # 先去重
                if nums[j] in dct:
                    continue
                dct.add(nums[j])
                nums[i], nums[j] = nums[j], nums[i]
                backtrack(i+1)
                nums[i], nums[j] = nums[j], nums[i]
        
        res = []
        backtrack(0)
        return res
```
The exchange method only needs a simple dictionary judgment, there are several differences:
1. Dictionary initialization is not a global concept in backtracking; because it is only necessary to judge whether the position of this backtracking is repeated
2. Dictionary elements are array elements, not subscript indexes
3. No need to introduce additional paths, directly operate and add the original array to the answer
Topic: 39. Combinatorial sums

Title: 40. Combinatorial sums II

Combination questions feel more like subsets, but not all subsets, but conditional subsets
1. Current element selection/unselection (suitable for reusing the same element), double branch
2. Use a for loop to verify all of the following
Topic: 17. Alphabet combinations of telephone numbers

Topic: 22. Bracket generation (note that there can be multiple backtracking conditions)

Topic: 79. Word Search (Classic)
dynamic programming

(Unplanned) Topic: 198. Robbery

Title: 213. House Robbery II

The maximum value of 2 cases: If the first house is stolen, the last house cannot be stolen, so the range of houses stolen is from the first house to the last second house; if the last house is stolen, the first house cannot be stolen, so the range of houses stolen is from the second house to the last house.

Title: 55. Jumping Game

Typical greedy , remember the method; very similar to Byte’s summer internship written test questions (the title is called “Byte Beat”)

Title: 45. Jumping Game II (Continuing Greedy Strategy from I)

Title: 62. Different paths

Topic: 5. The longest palindrome substring (it's a bit strange to put it here, the optimal solution to the horse-drawn cart is success)

Topic: 413. Arithmetic Sequence Division

It is very similar to a certain question, I remember that question can not only consider the adjacent 3 elements

Title: 91. Decoding Methods

The current situation is equal to the addition of two situations, but both situations need to be judged

Title: 139. Word Splitting

Double loop type dp, the second loop judges whether a previous state is satisfied, that is, the current value can only be obtained when a certain small condition is satisfied (the section of the enumerated value is in the dictionary)

Title: 300. Longest Increasing Subsequence

Similar to the above question, the second cycle judges each situation, that is, the current value can only be obtained when a certain small condition is satisfied (the end value is greater than the enumerated value)

Title: 673. Number of Longest Increasing Subsequences

It is more difficult. On the basis of the above, both the small loop and the large loop update the number dp array

Title: 1143. Longest Common Subsequence

The public subsequence problem must be memorized!

Title: 583. Deletion of Two Strings (Problem Transformation)

Topic: 72. Edit distance (difficult question, so far not)

Topic: 322. Changing Change

In the same case 139, 300double loops, the enumeration of the inner loop must have a relationship with the enumeration value of the outer loop

Title: 343. Integer Splitting

The problem of cutting the rope, both dp and greedy
bit operation

Topic: 201. Bitwise AND of ranges of numbers (longest common prefix)
other

Subject: 384. Shuffling Arrays

Topic: 202. Happy Numbers

Title: [149. The most points on a straight line](

Leekow Learning Plan—Basic Notes on Data Structure and Algorithm

data structure

algorithm

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