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Given the head of a linked list, remove the nth node from the end of the list and return its head. Follow up: Could you do this in one pass? Example 1: Input: head = [1,2,3,4,5], n = 2 Output: [1,2,3,5] Example 2: Input: head = , n = 1 Output:  Example 3: Input: head = [1,2], n = 1 Output:  Constraints: The number of nodes in the list is sz. 1
Given a positive integer n, you can apply one of the following operations: If n is even, replace n with n / 2. If n is odd, replace n with either n + 1 or n - 1. Return the minimum number of operations needed for n to become 1. Example 1: Input: n = 8 Output: 3 Explanation: 8 -> 4 -> 2 -> 1 Example 2: Input: n = 7 Output: 4 Explanation: 7 -> 8 -> 4 -> 2 -> 1 or 7 -> 6 -> 3 -> 2 -> 1 Example 3: I..
In a given integer array nums, there is always exactly one largest element. Find whether the largest element in the array is at least twice as much as every other number in the array. If it is, return the index of the largest element, otherwise return -1. Example 1: Input: nums = [3, 6, 1, 0] Output: 1 Explanation: 6 is the largest integer, and for every other number in the array x, 6 is more th..
Given two strings A and B of lowercase letters, return true if you can swap two letters in A so the result is equal to B, otherwise, return false. Swapping letters is defined as taking two indices i and j (0-indexed) such that i != j and swapping the characters at A[i] and A[j]. For example, swapping at indices 0 and 2 in "abcd" results in "cbad". Example 1: Input: A = "ab", B = "ba" Output: tru..
Given an array of integers arr, a lucky integer is an integer which has a frequency in the array equal to its value. Return a lucky integer in the array. If there are multiple lucky integers return the largest of them. If there is no lucky integer return -1. Example 1: Input: arr = [2,2,3,4] Output: 2 Explanation: The only lucky number in the array is 2 because frequency == 2. Example 2: Inpu..
Suppose an array sorted in ascending order is rotated at some pivot unknown to you beforehand. (i.e., [0,0,1,2,2,5,6] might become [2,5,6,0,0,1,2]). You are given a target value to search. If found in the array return true, otherwise return false. Example 1: Input: nums = [2,5,6,0,0,1,2], target = 0 Output: true Example 2: Input: nums = [2,5,6,0,0,1,2], target = 3 Output: false Follow up: This i..
Given a matrix A, return the transpose of A. The transpose of a matrix is the matrix flipped over it's main diagonal, switching the row and column indices of the matrix. Example 1: Input: [[1,2,3],[4,5,6],[7,8,9]] Output: [[1,4,7],[2,5,8],[3,6,9]] Example 2: Input: [[1,2,3],[4,5,6]] Output: [[1,4],[2,5],[3,6]] Note: 1
Given two non-negative integers num1 and num2 represented as strings, return the product of num1 and num2, also represented as a string. Note: You must not use any built-in BigInteger library or convert the inputs to integer directly. Example 1: Input: num1 = "2", num2 = "3" Output: "6" Example 2: Input: num1 = "123", num2 = "456" Output: "56088" Constraints: 1 123으로 바뀌게 됩니다. 그럼 이렇게 바꾼 숫자를 곱하고 s..
Given an array arr, replace every element in that array with the greatest element among the elements to its right, and replace the last element with -1. After doing so, return the array. Example 1: Input: arr = [17,18,5,4,6,1] Output: [18,6,6,6,1,-1] Constraints: 1
Given two arrays arr1 and arr2, the elements of arr2 are distinct, and all elements in arr2 are also in arr1. Sort the elements of arr1 such that the relative ordering of items in arr1 are the same as in arr2. Elements that don't appear in arr2 should be placed at the end of arr1 in ascending order. Example 1: Input: arr1 = [2,3,1,3,2,4,6,7,9,2,19], arr2 = [2,1,4,3,9,6] Output: [2,2,2,1,4,3,3,9,..
Given an integer array sorted in non-decreasing order, there is exactly one integer in the array that occurs more than 25% of the time. Return that integer. Example 1: Input: arr = [1,2,2,6,6,6,6,7,10] Output: 6 Constraints: 1
Given a m * n matrix of distinct numbers, return all lucky numbers in the matrix in any order. A lucky number is an element of the matrix such that it is the minimum element in its row and maximum in its column. Example 1: Input: matrix = [[3,7,8],[9,11,13],[15,16,17]] Output:  Explanation: 15 is the only lucky number since it is the minimum in its row and the maximum in its column Example 2..
Given a m * n matrix grid which is sorted in non-increasing order both row-wise and column-wise. Return the number of negative numbers in grid. Example 1: Input: grid = [[4,3,2,-1],[3,2,1,-1],[1,1,-1,-2],[-1,-1,-2,-3]] Output: 8 Explanation: There are 8 negatives number in the matrix. Example 2: Input: grid = [[3,2],[1,0]] Output: 0 Example 3: Input: grid = [[1,-1],[-1,-1]] Output: 3 Example 4: ..
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 1: Input: n = 4 Output: 4 Explanation: T_3 = 0 + 1 + 1 = 2 T_4 = 1 + 1 + 2 = 4 Example 2: Input: n = 25 Output: 1389537 Constraints: 0
The Fibonacci numbers, commonly denoted F(n) form a sequence, called the Fibonacci sequence, such that each number is the sum of the two preceding ones, starting from 0 and 1. That is, F(0) = 0, F(1) = 1 F(N) = F(N - 1) + F(N - 2), for N > 1. Given N, calculate F(N). Example 1: Input: 2 Output: 1 Explanation: F(2) = F(1) + F(0) = 1 + 0 = 1. Example 2: Input: 3 Output: 2 Explanation: F(3) = F(2) ..
You are given a license key represented as a string S which consists only alphanumeric character and dashes. The string is separated into N+1 groups by N dashes. Given a number K, we would want to reformat the strings such that each group contains exactly K characters, except for the first group which could be shorter than K, but still must contain at least one character. Furthermore, there must..
Given an array of integers and an integer k, find out whether there are two distinct indices i and j in the array such that nums[i] = nums[j] and the absolute difference between i and j is at most k. Example 1: Input: nums = [1,2,3,1], k = 3 Output: true Example 2: Input: nums = [1,0,1,1], k = 1 Output: true Example 3: Input: nums = [1,2,3,1,2,3], k = 2 Output: false Solution class Solution: def..
Given an array of integers, find if the array contains any duplicates. Your function should return true if any value appears at least twice in the array, and it should return false if every element is distinct. Example 1: Input: [1,2,3,1] Output: true Example 2: Input: [1,2,3,4] Output: false Example 3: Input: [1,1,1,3,3,4,3,2,4,2] Output: true Solution from collections import Counter class Solu..
Given a list of non-negative integers nums, arrange them such that they form the largest number. Note: The result may be very large, so you need to return a string instead of an integer. Example 1: Input: nums = [10,2] Output: "210" Example 2: Input: nums = [3,30,34,5,9] Output: "9534330" Example 3: Input: nums =  Output: "1" Example 4: Input: nums =  Output: "10" Constraints: 1
Given an array nums and a value val, remove all instances of that value in-place and return the new length. Do not allocate extra space for another array, you must do this by modifying the input array in-place with O(1) extra memory. The order of elements can be changed. It doesn't matter what you leave beyond the new length. Clarification: Confused why the returned value is an integer but your ..
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. Example 1: Input: k = 3, n = 7 Output: [[1,2,4]] Explanation: 1 + 2 + 4..
Reverse a singly linked list. Example: Input: 1->2->3->4->5->NULL Output: 5->4->3->2->1->NULL Follow up: A linked list can be reversed either iteratively or recursively. Could you implement both? Solution # Definition for singly-linked list. # class ListNode: # def __init__(self, val=0, next=None): # self.val = val # self.next = next class Solution: def reverseList(self, head: ListNode) -> ListN..
Given words first and second, consider occurrences in some text of the form "first second third", where second comes immediately after first, and third comes immediately after second. For each such occurrence, add "third" to the answer, and return the answer. Example 1: Input: text = "alice is a good girl she is a good student", first = "a", second = "good" Output: ["girl","student"] Example 2: ..
Given a string, sort it in decreasing order based on the frequency of characters. Example 1: Input: "tree" Output: "eert" Explanation: 'e' appears twice while 'r' and 't' both appear once. So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also a valid answer. Example 2: Input: "cccaaa" Output: "cccaaa" Explanation: Both 'c' and 'a' appear three times, so "aaaccc" is also a valid an..
Find the kth largest element in an unsorted array. Note that it is the kth largest element in the sorted order, not the kth distinct element. Example 1: Input: [3,2,1,5,6,4] and k = 2 Output: 5 Example 2: Input: [3,2,3,1,2,4,5,5,6] and k = 4 Output: 4 Note: You may assume k is always valid, 1 ≤ k ≤ array's length. Solution class Solution: def findKthLargest(self, nums: List[int], k: int) -> int:..
Given a non-empty array of integers, return the k most frequent elements. Example 1: Input: nums = [1,1,1,2,2,3], k = 2 Output: [1,2] Example 2: Input: nums = , k = 1 Output:  Note: You may assume k is always valid, 1 ≤ k ≤ number of unique elements. Your algorithm's time complexity must be better than O(n log n), where n is the array's size. It's guaranteed that the answer is unique, in o..
Given an array of integers, 1 ≤ a[i] ≤ n (n = size of array), some elements appear twice and others appear once. Find all the elements that appear twice in this array. Could you do it without extra space and in O(n) runtime? Example: Input: [4,3,2,7,8,2,3,1] Output: [2,3] Solution from collections import Counter class Solution: def findDuplicates(self, nums: List[int]) -> List[int]: cnt_items = ..
Given an integer, return its base 7 string representation. Example 1: Input: 100 Output: "202" Example 2: Input: -7 Output: "-10" Note: The input will be in range of [-1e7, 1e7]. Solution class Solution: def convertToBase7(self, num: int) -> str: if num == 0: return str(num) sign = 1 if num < 0: sign = -1 num = abs(num) answer = '' while num: answer = str(num % 7) + answer num //= 7 return ('-' ..
Given the array nums consisting of 2n elements in the form [x1,x2,...,xn,y1,y2,...,yn]. Return the array in the form [x1,y1,x2,y2,...,xn,yn]. Example 1: Input: nums = [2,5,1,3,4,7], n = 3 Output: [2,3,5,4,1,7] Explanation: Since x1=2, x2=5, x3=1, y1=3, y2=4, y3=7 then the answer is [2,3,5,4,1,7]. Example 2: Input: nums = [1,2,3,4,4,3,2,1], n = 4 Output: [1,4,2,3,3,2,4,1] Example 3: Input: nums =..
An array is monotonic if it is either monotone increasing or monotone decreasing. An array A is monotone increasing if for all i