Leetcode.com 2021-09-14
🟢 917.reverse-only-letters

🏷️ Tags
#two_pointers #string

Description
Given a string s, reverse the string according to the following rules:


All the characters that are not English letters remain in the same position.
All the English letters (lowercase or uppercase) should be reversed.


Return s after reversing it.

Example

Input: s = "ab-cd"
Output: "dc-ba"

Leetcode-cn.com 2021-09-16
🔴 212.word-search-ii

🏷️ Tags
#trie #array #string #backtracking #matrix

Description
给定一个 m x n 二维字符网格 board 和一个单词（字符串）列表 words，找出所有同时在二维网格和字典中出现的单词。

单词必须按照字母顺序，通过 相邻的单元格 内的字母构成，其中“相邻”单元格是那些水平相邻或垂直相邻的单元格。同一个单元格内的字母在一个单词中不允许被重复使用。

 

Example

输入：board = [["o","a","a","n"],["e","t","a","e"],["i","h","k","r"],["i","f","l","v"]], words = ["oath","pea","eat","rain"]
输出：["eat","oath"]

Leetcode.com 2021-09-15
🟡 978.longest-turbulent-subarray

🏷️ Tags
#array #dynamic_programming #sliding_window

Description
Given an integer array arr, return the length of a maximum size turbulent subarray of arr.

A subarray is turbulent if the comparison sign flips between each adjacent pair of elements in the subarray.

More formally, a subarray [arr[i], arr[i + 1], ..., arr[j]] of arr is said to be turbulent if and only if:


For i <= k < j:


arr[k] > arr[k + 1] when k is odd, and
arr[k] < arr[k + 1] when k is even.


Or, for i <= k < j:

arr[k] > arr[k + 1] when k is even, and
arr[k] < arr[k + 1] when k is odd.




Example

Input: arr = [9,4,2,10,7,8,8,1,9]
Output: 5
Explanation: arr[1] > arr[2] < arr[3] > arr[4] < arr[5]

Leetcode-cn.com 2021-09-17
🟡 36.valid-sudoku

🏷️ Tags
#array #hash_table #matrix

Description
请你判断一个 9x9 的数独是否有效。只需要 根据以下规则 ，验证已经填入的数字是否有效即可。


数字 1-9 在每一行只能出现一次。
数字 1-9 在每一列只能出现一次。
数字 1-9 在每一个以粗实线分隔的 3x3 宫内只能出现一次。（请参考Example

输入：board = 
[["5","3",".",".","7",".",".",".","."]
,["6",".",".","1","9","5",".",".","."]
,[".","9","8",".",".",".",".","6","."]
,["8",".",".",".","6",".",".",".","3"]
,["4",".",".","8",".","3",".",".","1"]
,["7",".",".",".","2",".",".",".","6"]
,[".","6",".",".",".",".","2","8","."]
,[".",".",".","4","1","9",".",".","5"]
,[".",".",".",".","8",".",".","7","9"]]
输出：true

Leetcode.com 2021-09-16
🟡 54.spiral-matrix

🏷️ Tags
#array #matrix #simulation

Description
Given an m x n matrix, return all elements of the matrix in spiral order.

Example

Input: matrix = [[1,2,3],[4,5,6],[7,8,9]]
Output: [1,2,3,6,9,8,7,4,5]

Leetcode-cn.com 2021-09-18
🟢 292.nim-game

🏷️ Tags
#brainteaser #math #game_theory

Description
你和你的朋友，两个人一起玩 Nim 游戏：


桌子上有一堆石头。
你们轮流进行自己的回合，你作为先手。
每一回合，轮到的人拿掉 1 - 3 块石头。
拿掉最后一块石头的人就是获胜者。


假设你们每一步都是最优解。请编写一个函数，来判断你是否可以在给定石头数量为 n 的情况下赢得游戏。如果可以赢，返回 true；否则，返回 false 。

 

Example

输入：n = 4
输出：false 
解释：如果堆中有 4 块石头，那么你永远不会赢得比赛；
     因为无论你拿走 1 块、2 块 还是 3 块石头，最后一块石头总是会被你的朋友拿走。

Leetcode.com 2021-09-17
🟢 350.intersection-of-two-arrays-ii

🏷️ Tags
#array #hash_table #two_pointers #binary_search #sorting

Description
Given two integer arrays nums1 and nums2, return an array of their intersection. Each element in the result must appear as many times as it shows in both arrays and you may return the result in any order.

Example

Input: nums1 = [1,2,2,1], nums2 = [2,2]
Output: [2,2]

Leetcode-cn.com 2021-09-19
🟡 650.2-keys-keyboard

🏷️ Tags
#math #dynamic_programming

Description
最初记事本上只有一个字符 'A' 。你每次可以对这个记事本进行两种操作：


Copy All（复制全部）：复制这个记事本中的所有字符（不允许仅复制部分字符）。
Paste（粘贴）：粘贴 上一次 复制的字符。


给你一个数字 n ，你需要使用最少的操作次数，在记事本上输出 恰好 n 个 'A' 。返回能够打印出 n 个 'A' 的最少操作次数。

Example

输入：3
输出：3
解释：
最初, 只有一个字符 'A'。
第 1 步, 使用 Copy All 操作。
第 2 步, 使用 Paste 操作来获得 'AA'。
第 3 步, 使用 Paste 操作来获得 'AAA'。

Leetcode.com 2021-09-18
🔴 282.expression-add-operators

🏷️ Tags
#math #string #backtracking

Description
Given a string num that contains only digits and an integer target, return all possibilities to add the binary operators '+', '-', or '*' between the digits of num so that the resultant expression evaluates to the target value.

Example

Input: num = "123", target = 6
Output: ["1*2*3","1+2+3"]

Leetcode-cn.com 2021-09-20
🟡 673.number-of-longest-increasing-subsequence

🏷️ Tags
#binary_indexed_tree #segment_tree #array #dynamic_programming

Description
给定一个未排序的整数数组，找到最长递增子序列的个数。

Example

输入: [1,3,5,4,7]
输出: 2
解释: 有两个最长递增子序列，分别是 [1, 3, 4, 7] 和[1, 3, 5, 7]。

Leetcode.com 2021-09-19
🔴 115.distinct-subsequences

🏷️ Tags
#string #dynamic_programming

Description
Given two strings s and t, return the number of distinct subsequences of s which equals t.

A string's subsequence is a new string formed from the original string by deleting some (can be none) of the characters without disturbing the remaining characters' relative positions. (i.e., "ACE" is a subsequence of "ABCDE" while "AEC" is not).

It is guaranteed the answer fits on a 32-bit signed integer.

Example

Input: s = "rabbbit", t = "rabbit"
Output: 3
Explanation:
As shown below, there are 3 ways you can generate "rabbit" from S.
rabbbit
rabbbit
rabbbit

Leetcode-cn.com 2021-09-21
🟢 58.length-of-last-word

🏷️ Tags
#string

Description
给你一个字符串 s，由若干单词组成，单词前后用一些空格字符隔开。返回字符串中最后一个单词的长度。

单词 是指仅由字母组成、不包含任何空格字符的最大子字符串。

Example

输入：s = "Hello World"
输出：5

Leetcode.com 2021-09-20
🟢 1275.find-winner-on-a-tic-tac-toe-game

🏷️ Tags
#array #hash_table #matrix #simulation

Description
Tic-tac-toe is played by two players A and B on a 3 x 3 grid.

Here are the rules of Tic-Tac-Toe:


Players take turns placing characters into empty squares (" ").
The first player A always places "X" characters, while the second player B always places "O" characters.
"X" and "O" characters are always placed into empty squares, never on filled ones.
The game ends when there are 3 of the same (non-empty) character filling any row, column, or diagonal.
The game also ends if all squares are non-empty.
No more moves can be played if the game is over.


Given an array moves where each element is another array of size 2 corresponding to the row and column of the grid where they mark their respective character in the order in which A and B play.

Return the winner of the game if it exists (A or B), in case the game ends in a draw return "Draw", if there are still movements to play return "Pending".

You can assume that moves is valid (It follows the rules of Tic-Tac-Toe), the grid is initially empty and A will play first.

Example

Input: moves = [[0,0],[2,0],[1,1],[2,1],[2,2]]
Output: "A"
Explanation: "A" wins, he always plays first.
"X  "    "X  "    "X  "    "X  "    "X  "
"   " -> "   " -> " X " -> " X " -> " X "
"   "    "O  "    "O  "    "OO "    "OOX"

Leetcode-cn.com 2021-09-22
🟡 725.split-linked-list-in-parts

🏷️ Tags
#linked_list

Description
给定一个头结点为 root 的链表, 编写一个函数以将链表分隔为 k 个连续的部分。

每部分的长度应该尽可能的相等: 任意两部分的长度差距不能超过 1，也就是说可能有些部分为 null。

这k个部分应该按照在链表中出现的顺序进行输出，并且排在前面的部分的长度应该大于或等于后面的长度。

返回一个符合上述规则的链表的列表。

举例： 1->2->3->4, k = 5 // 5 结果 [ [1], [2], [3], [4], null ]

Example

输入: 
root = [1, 2, 3], k = 5
输出: [[1],[2],[3],[],[]]
解释:
输入输出各部分都应该是链表，而不是数组。
例如, 输入的结点 root 的 val= 1, root.next.val = 2, \root.next.next.val = 3, 且 root.next.next.next = null。
第一个输出 output[0] 是 output[0].val = 1, output[0].next = null。
最后一个元素 output[4] 为 null, 它代表了最后一个部分为空链表。

Leetcode.com 2021-09-21
🟢 485.max-consecutive-ones

🏷️ Tags
#array

Description
Given a binary array nums, return the maximum number of consecutive 1's in the array.

Example

Input: nums = [1,1,0,1,1,1]
Output: 3
Explanation: The first two digits or the last three digits are consecutive 1s. The maximum number of consecutive 1s is 3.

Leetcode-cn.com 2021-09-23
🟢 326.power-of-three

🏷️ Tags
#recursion #math

Description
给定一个整数，写一个函数来判断它是否是 3 的幂次方。如果是，返回 true ；否则，返回 false 。

整数 n 是 3 的幂次方需满足：存在整数 x 使得 n == 3x

 

Example

输入：n = 27
输出：true

Leetcode.com 2021-09-22
🟡 1239.maximum-length-of-a-concatenated-string-with-unique-characters

🏷️ Tags
#bit_manipulation #array #string #backtracking

Description
Given an array of strings arr. String s is a concatenation of a sub-sequence of arr which have unique characters.

Return the maximum possible length of s.

Example

Input: arr = ["un","iq","ue"]
Output: 4
Explanation: All possible concatenations are "","un","iq","ue","uniq" and "ique".
Maximum length is 4.

Leetcode.com 2021-09-23
🟡 1328.break-a-palindrome

🏷️ Tags
#greedy #string

Description
Given a palindromic string of lowercase English letters palindrome, replace exactly one character with any lowercase English letter so that the resulting string is not a palindrome and that it is the lexicographically smallest one possible.

Return the resulting string. If there is no way to replace a character to make it not a palindrome, return an empty string.

A string a is lexicographically smaller than a string b (of the same length) if in the first position where a and b differ, a has a character strictly smaller than the corresponding character in b. For example, "abcc" is lexicographically smaller than "abcd" because the first position they differ is at the fourth character, and 'c' is smaller than 'd'.

Example

Input: palindrome = "abccba"
Output: "aaccba"
Explanation: There are many ways to make "abccba" not a palindrome, such as "zbccba", "aaccba", and "abacba".
Of all the ways, "aaccba" is the lexicographically smallest.

Leetcode-cn.com 2021-09-25
🟡 583.delete-operation-for-two-strings

🏷️ Tags
#string #dynamic_programming

Description
给定两个单词 word1 和 word2，找到使得 word1 和 word2 相同所需的最小步数，每步可以删除任意一个字符串中的一个字符。

Example

输入: "sea", "eat"
输出: 2
解释: 第一步将"sea"变为"ea"，第二步将"eat"变为"ea"

Leetcode.com 2021-09-24
🟢 1137.n-th-tribonacci-number

🏷️ Tags
#memoization #math #dynamic_programming

Description
The Tribonacci sequence Tn is defined as follows:

T0 = 0, T1 = 1, T2 = 1, and Tn+3 = Tn + Tn+1 + Tn+2 for n >= 0.

Given n, return the value of Tn.

Example

Input: n = 4
Output: 4
Explanation:
T_3 = 0 + 1 + 1 = 2
T_4 = 1 + 1 + 2 = 4