Leetcode

Problem Find all valid combinations of k numbers that sum up to n such that the following conditions are true: Only numbers 1 through 9 are used. Each number is used at most once. Return a list of all possible valid combinations. The list must not contain the same combination twice, and the combinations may be returned in any order. Examples Example 1: Input: k = 3, n = 7 Output: [ [1,2,4] ] Explanation: 1 + 2 + 4 = 7 There are no other valid combinations. ...

Problem A trie (pronounced as “try”) or prefix tree is a tree data structure used to efficiently store and retrieve keys in a dataset of strings. There are various applications of this data structure, such as autocomplete and spellchecker. Implement the Trie class: Trie() Initializes the trie object. void insert(String word) Inserts the string word into the trie. boolean search(String word) Returns true if the string word is in the trie (i.e., was inserted before), and false otherwise. boolean startsWith(String prefix) Returns true if there is a previously inserted string word that has the prefix prefix, and false otherwise. Examples Example 1: ...

Problem Given an array of positive integers nums and a positive integer target, return the minimal length of a subarray whose sum is greater than or equal to target. If there is no such subarray, return 0 instead. OR Given an array of positive integers a and a positive number K, find the length of the smallest contiguous subarray whose sum is greater than or equal to K. Return 0 if no such subarray exists. ...

Problem Given a text file file.txt that contains a list of phone numbers (one per line), write a one-liner bash script to print all valid phone numbers. You may assume that a valid phone number must appear in one of the following two formats: (xxx) xxx-xxxx or xxx-xxx-xxxx. (x means a digit) You may also assume each line in the text file must not contain leading or trailing white spaces. ...

Problem Given an m x n 2D binary grid grid which represents a map of '1's (land) and '0's (water), return the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water. Examples Example 1: Input: grid = [ ["1","1","1","1","0"], ["1","1","0","1","0"], ["1","1","0","0","0"], ["0","0","0","0","0"] ] Output: 1 ...

Word Frequency From File Problem Write a bash script to calculate the frequency of each word in a text file words.txt. For simplicity sake, you may assume: words.txt contains only lowercase characters and space ' ' characters. Each word must consist of lowercase characters only. Words are separated by one or more whitespace characters. Examples Example: Assume that words.txt has the following content: ...

Problem The API: int read4(char *buf) reads 4 characters at a time from a file. The return value is the actual number of characters read. For example, it returns 3 if there is only 3 characters left in the file. By using the read4 API, implement the function int read(char *buf, int n) that reads n characters from the file. Examples Example 1 Input: "filetestbuffer" read(6) read(5) read(4) read(3) read(2) read(1) read(10) Output: 6, buf = "filete" 5, buf = "stbuf" 3, buf = "fer" 0, buf = "" 0, buf = "" 0, buf = "" 0, buf = "" ...

Problem Given a file and assume that you can only read the file using a given method read4, implement a method to read n characters. Method read4: The API read4 reads 4 consecutive characters from the file, then writes those characters into the buffer array buf. The return value is the number of actual characters read. Note that read4() has its own file pointer, much like FILE *fp in C. ...

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: Insertion sort iterates, consuming one input element each repetition and growing a sorted output list. 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. It repeats until no input elements remain. The following is a graphical example of the insertion sort algorithm. ...

Problem You are given the head of a singly linked-list. The list can be represented as: $$ L_0 → L_1 → … → L_{n - 1} → L_n $$ Reorder the list to be on the following form:: $$ L_0 → L_n → L1 → L_{n - 1} → L2 → L_{n - 2} → … $$ Examples Example 1: --- title: Input List --- graph LR A1[1] --> B2[2] --> C3[3] --> D4[4] ...