2021-12-11
878. Nth Magical Number

Topic: Math, Binary Search
Difficulty: Hard

Problem:
A positive integer is magical if it is divisible by either a or b.

Given the three integers n, a, and b, return the n^th magical number. Since the answer may be very large, return it modulo 10^9 + 7.

Example 1:

Input: n = 1, a = 2, b = 3
Output: 2

Example 2:

Input: n = 4, a = 2, b = 3
Output: 6

Example 3:

Input: n = 5, a = 2, b = 4
Output: 10

Example 4:

Input: n = 3, a = 6, b = 4
Output: 8

Constraints:

1 <= n <= 10^92 <= a, b <= 4 * 10^4

2021-12-12
416. Partition Equal Subset Sum

Topic: Array, Dynamic Programming
Difficulty: Medium

Problem:
Given a non-empty array nums containing only positive integers, find if the array can be partitioned into two subsets such that the sum of elements in both subsets is equal.

Example 1:

Input: nums = [1,5,11,5]
Output: true
Explanation: The array can be partitioned as [1, 5, 5] and [11].

Example 2:

Input: nums = [1,2,3,5]
Output: false
Explanation: The array cannot be partitioned into equal sum subsets.

Constraints:

• 1 <= nums.length <= 200
• 1 <= nums[i] <= 100

2021-12-13
1446. Consecutive Characters

Topic: String
Difficulty: Easy

Problem:
The power of the string is the maximum length of a non-empty substring that contains only one unique character.

Given a string s, return the power of s.

Example 1:

Input: s = "leetcode"
Output: 2
Explanation: The substring "ee" is of length 2 with the character 'e' only.

Example 2:

Input: s = "abbcccddddeeeeedcba"
Output: 5
Explanation: The substring "eeeee" is of length 5 with the character 'e' only.

Example 3:

Input: s = "triplepillooooow"
Output: 5

Example 4:

Input: s = "hooraaaaaaaaaaay"
Output: 11

Example 5:

Input: s = "tourist"
Output: 1

Constraints:

• 1 <= s.length <= 500
• s consists of only lowercase English letters.

2021-12-14
938. Range Sum of BST

Topic: Tree, Depth-First Search, Binary Search Tree, Binary Tree
Difficulty: Easy

Problem:
Given the root node of a binary search tree and two integers low and high, return the sum of values of all nodes with a value in the inclusive range [low, high].

Example 1:

Image: https://assets.leetcode.com/uploads/2020/11/05/bst1.jpg

Input: root = [10,5,15,3,7,null,18], low = 7, high = 15
Output: 32
Explanation: Nodes 7, 10, and 15 are in the range [7, 15]. 7 + 10 + 15 = 32.

Example 2:

Image: https://assets.leetcode.com/uploads/2020/11/05/bst2.jpg

Input: root = [10,5,15,3,7,13,18,1,null,6], low = 6, high = 10
Output: 23
Explanation: Nodes 6, 7, and 10 are in the range [6, 10]. 6 + 7 + 10 = 23.

Constraints:

• The number of nodes in the tree is in the range [1, 2 * 10^4].
• 1 <= Node.val <= 10^5
• 1 <= low <= high <= 10^5
• All Node.val are unique.

2021-12-15
147. Insertion Sort List

Topic: Linked List, Sorting
Difficulty: Medium

Problem:
Given the head of a singly linked list, sort the list using insertion sort, and return the sorted list's head.

The steps of the insertion sort algorithm:

1. Insertion sort iterates, consuming one input element each repetition and growing a sorted output list.
2. At each iteration, insertion sort removes one element from the input data, finds the location it belongs within the sorted list and inserts it there.
3. It repeats until no input elements remain.

The following is a graphical example of the insertion sort algorithm. The partially sorted list (black) initially contains only the first element in the list. One element (red) is removed from the input data and inserted in-place into the sorted list with each iteration.

Image: https://upload.wikimedia.org/wikipedia/commons/0/0f/Insertion-sort-example-300px.gif

Example 1:

Image: https://assets.leetcode.com/uploads/2021/03/04/sort1linked-list.jpg

Input: head = [4,2,1,3]
Output: [1,2,3,4]

Example 2:

Image: https://assets.leetcode.com/uploads/2021/03/04/sort2linked-list.jpg

Input: head = [-1,5,3,4,0]
Output: [-1,0,3,4,5]

Constraints:

• The number of nodes in the list is in the range [1, 5000].
• -5000 <= Node.val <= 5000

2021-12-16
310. Minimum Height Trees

Topic: Depth-First Search, Breadth-First Search, Graph, Topological Sort
Difficulty: Medium

Problem:
A tree is an undirected graph in which any two vertices are connected by exactly one path. In other words, any connected graph without simple cycles is a tree.

Given a tree of n nodes labelled from 0 to n - 1, and an array of n - 1 edges where edges[i] = [a_i, b_i] indicates that there is an undirected edge between the two nodes a_i and b_i in the tree, you can choose any node of the tree as the root. When you select a node x as the root, the result tree has height h. Among all possible rooted trees, those with minimum height (i.e. min(h))  are called minimum height trees (MHTs).

Return a list of all MHTs' root labels. You can return the answer in any order.

The height of a rooted tree is the number of edges on the longest downward path between the root and a leaf.

Example 1:

Image: https://assets.leetcode.com/uploads/2020/09/01/e1.jpg

Input: n = 4, edges = [[1,0],[1,2],[1,3]]
Output: [1]
Explanation: As shown, the height of the tree is 1 when the root is the node with label 1 which is the only MHT.

Example 2:

Image: https://assets.leetcode.com/uploads/2020/09/01/e2.jpg

Input: n = 6, edges = [[3,0],[3,1],[3,2],[3,4],[5,4]]
Output: [3,4]

Example 3:

Input: n = 1, edges = []
Output: [0]

Example 4:

Input: n = 2, edges = [[0,1]]
Output: [0,1]

Constraints:

• 1 <= n <= 2 * 10^4
• edges.length == n - 1
• 0 <= a_i, b_i < n
• a_i != b_i
• All the pairs (a_i, b_i) are distinct.
• The given input is guaranteed to be a tree and there will be no repeated edges.

2021-12-17
221. Maximal Square

Topic: Array, Dynamic Programming, Matrix
Difficulty: Medium

Problem:
Given an m x n binary matrix filled with 0's and 1's, find the largest square containing only 1's and return its area.

Example 1:

Image: https://assets.leetcode.com/uploads/2020/11/26/max1grid.jpg

Input: matrix = [["1","0","1","0","0"],["1","0","1","1","1"],["1","1","1","1","1"],["1","0","0","1","0"]]
Output: 4

Example 2:

Image: https://assets.leetcode.com/uploads/2020/11/26/max2grid.jpg

Input: matrix = [["0","1"],["1","0"]]
Output: 1

Example 3:

Input: matrix = [["0"]]
Output: 0

Constraints:

• m == matrix.length
• n == matrix[i].length
• 1 <= m, n <= 300
• matrix[i][j] is '0' or '1'.

2021-12-18
902. Numbers At Most N Given Digit Set

Topic: Array, Math, Binary Search, Dynamic Programming
Difficulty: Hard

Problem:
Given an array of digits which is sorted in non-decreasing order. You can write numbers using each digits[i] as many times as we want. For example, if digits = ['1','3','5'], we may write numbers such as '13', '551', and '1351315'.

Return the number of positive integers that can be generated that are less than or equal to a given integer n.

Example 1:

Input: digits = ["1","3","5","7"], n = 100
Output: 20
Explanation: 
The 20 numbers that can be written are:
1, 3, 5, 7, 11, 13, 15, 17, 31, 33, 35, 37, 51, 53, 55, 57, 71, 73, 75, 77.

Example 2:

Input: digits = ["1","4","9"], n = 1000000000
Output: 29523
Explanation: 
We can write 3 one digit numbers, 9 two digit numbers, 27 three digit numbers,
81 four digit numbers, 243 five digit numbers, 729 six digit numbers,
2187 seven digit numbers, 6561 eight digit numbers, and 19683 nine digit numbers.
In total, this is 29523 integers that can be written using the digits array.

Example 3:

Input: digits = ["7"], n = 8
Output: 1

Constraints:

• 1 <= digits.length <= 9
• digits[i].length == 1
• digits[i] is a digit from '1' to '9'.
• All the values in digits are unique.
• digits is sorted in non-decreasing order.
• 1 <= n <= 10^9