problems/

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Problems

These are some hands-on problem, and you can use a shared Google Doc to read and write code together. They don't require the interviewee to have an IDE or coding environment, and the main part of your discussion should be Algorithm.

KNN

Implement KNN Algorithm.

Decode String

Given an encoded string, return its decoded string.

The encoding rule is: k[encoded_string], where the encoded_string inside the square brackets is being repeated exactly k times. Note that k is guaranteed to be a positive integer.

You may assume that the input string is always valid; there are no extra white spaces, square brackets are well-formed, etc. Furthermore, you may assume that the original data does not contain any digits and that digits are only for those repeat numbers, k. For example, there will not be input like 3a or 2[4].

The test cases are generated so that the length of the output will never exceed 10^5.

Example 1:

Input: s = "3[a]2[bc]"
Output: "aaabcbc"

Example 2:

Input: s = "3[a2[c]]"
Output: "accaccacc"

Example 3:

Input: s = "2[abc]3[cd]ef"
Output: "abcabccdcdcdef"

Constraints:

• 1 <= s.length <= 30
• s consists of lowercase English letters, digits, and square brackets '[]'.
• s is guaranteed to be a valid input.
• All the integers in s are in the range [1, 300].

LeetCode

Find All Groups of Farmland

You are given a 0-indexed m x n binary matrix land where a 0 represents a hectare of forested land and a 1 represents a hectare of farmland.

To keep the land organized, there are designated rectangular areas of hectares that consist entirely of farmland. These rectangular areas are called groups. No two groups are adjacent, meaning farmland in one group is not four-directionally adjacent to another farmland in a different group.

land can be represented by a coordinate system where the top left corner of land is (0, 0) and the bottom right corner of land is (m-1, n-1). Find the coordinates of the top left and bottom right corner of each group of farmland. A group of farmland with a top left corner at (r1, c1) and a bottom right corner at (r2, c2) is represented by the 4-length array [r1, c1, r2, c2].

Return a 2D array containing the 4-length arrays described above for each group of farmland in land. If there are no groups of farmland, return an empty array. You may return the answer in any order.

LeetCode

Number of Islands

Given an m x n 2D binary 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.

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

Example 2:

Input: grid = [
  ["1","1","0","0","0"],
  ["1","1","0","0","0"],
  ["0","0","1","0","0"],
  ["0","0","0","1","1"]
]
Output: 3

Constraints:

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

LeetCode

Shuffle

You have an array with n-items (A). We want to partition it into k-subarrays that each of them has n/k items, and each element of A appears precisely once. The order of these subarrays must not be the same as the A.

We know that: n % k == 0

• With duplication or without duplication?

For example:

A = [1, 2, 3, 4]

k = 2

we don't accept the following solution:

A1 = {1, 2}
A2 = {3, 4}

but we accept the following solution:

A1 = [1, 3]
A2 = [2, 4]

Coins

We have n amount of money and our country have the following coins:

• coin-1
• coin-5
• coin-7
• coin-10

We want to have this money with minimum number of coins. What is the minimum? For example:

• 2 = 2 x coin-1
• 5 = 1 x coin-5
• 6 = 1 x coin-5 + 1 x coin-1

Bulb Switcher

There are n bulbs that are initially off. You first turn on all the bulbs. Then, you turn off every second bulb. On the third round, you toggle every third bulb (turning on if it's off or turning off if it's on). For the i-th round, you toggle every i bulb. For the n-th round, you only toggle the last bulb. Find how many bulbs are on after n rounds.

Example:

Input: 3
Output: 1
Explanation:
At first, the three bulbs are [off, off, off].
After first round, the three bulbs are [on, on, on].
After second round, the three bulbs are [on, off, on].
After third round, the three bulbs are [on, off, off].

So you should return 1, because there is only one bulb is on.

LeetCode

Happy Number

Write an algorithm to determine if a number n is "happy".

A happy number is a number defined by the following process: Starting with any positive integer, replace the number by the sum of the squares of its digits, and repeat the process until the number equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1. Those numbers for which this process ends in 1 are happy numbers.

Return True if n is a happy number, and False if not.

Example:

Input: 19
Output: true
Explanation:
1^2 + 9^2 = 82
8^2 + 2^2 = 68
6^2 + 8^2 = 100
1^2 + 0^2 + 0^2 = 1

LeetCode

Rotate Image

You are given an n x n 2D matrix representing an image. Rotate the image by 90 degrees (clockwise).

Given input matrix =
[
  [1,2,3],
  [4,5,6],
  [7,8,9]
],

rotate the input matrix in-place such that it becomes:
[
  [7,4,1],
  [8,5,2],
  [9,6,3]
]

Given input matrix =
[
  [ 5, 1, 9,11],
  [ 2, 4, 8,10],
  [13, 3, 6, 7],
  [15,14,12,16]
],

rotate the input matrix in-place such that it becomes:
[
  [15,13, 2, 5],
  [14, 3, 4, 1],
  [12, 6, 8, 9],
  [16, 7,10,11]
]

LeetCode

Snappfood

We have motorcycles and restaurants. Motorcycles deliver foods to peoples from restaurants. How we can schedule this delivery process?

Search a 2D Matrix

You are given a m x n integer matrix matrix with the following two properties:

• Each row is sorted in non-decreasing order.
• The first integer of each row is greater than the last integer of the previous row.

Given an integer target, return true if target is in matrix or false otherwise.

You must write a solution in O(log(m * n)) time complexity.

Example 1:

Input: matrix = [[1,3,5,7],[10,11,16,20],[23,30,34,60]], target = 3
Output: true

Example 2:

Input: matrix = [[1,3,5,7],[10,11,16,20],[23,30,34,60]], target = 13
Output: false

Constraints:

• m == matrix.length
• n == matrix[i].length
• 1 <= m, n <= 100
• -10^4 <= matrix[i][j], target <= 10^4

Search a 2D Matrix II

Write an efficient algorithm that searches for a value target in an m x n integer matrix. This matrix has the following properties:

• Integers in each row are sorted in ascending from left to right.
• Integers in each column are sorted in ascending from top to bottom.

Input: matrix = [[1,4,7,11,15],[2,5,8,12,19],[3,6,9,16,22],[10,13,14,17,24],[18,21,23,26,30]], target = 5
Output: true

Input: matrix = [[1,4,7,11,15],[2,5,8,12,19],[3,6,9,16,22],[10,13,14,17,24],[18,21,23,26,30]], target = 20
Output: false

Constraints:

• m == matrix.length
• n == matrix[i].length
• 1 <= n, m <= 300
• -109 <= matrix[i][j] <= 109
• All the integers in each row are sorted in ascending order.
• All the integers in each column are sorted in ascending order.
• -109 <= target <= 109

LeetCode

Longest Palindromic Substring

Given a string s, return the longest palindromic substring in s.

Example 1:

Input: s = "babad"
Output: "bab"
Explanation: "aba" is also a valid answer.

Example 2:

Input: s = "cbbd"
Output: "bb"

LeetCode

K-th Smallest Element in a Sorted Matrix

LeetCode

Merge k Sorted Lists

You are given an array of k linked-lists lists, each linked-list is sorted in ascending order. Merge all the linked-lists into one sorted linked-list and return it.

Example 1:

Input: lists = [[1,4,5],[1,3,4],[2,6]]
Output: [1,1,2,3,4,4,5,6]
Explanation: The linked-lists are:
[
  1->4->5,
  1->3->4,
  2->6
]
merging them into one sorted list:
1->1->2->3->4->4->5->6

Example 2:

Input: lists = []
Output: []

Example 3:

Input: lists = [[]]
Output: []

LeetCode

K Nearest Neighbor

We have n points and one reference point. Each point has x and y coordinates. We want to find k the nearest points to the reference point.

For example:

import dataclasses

@dataclasses.dataclass()
class Point:
  x: float
  y: float

points = [
  Point(0, 0), Point(0, 1), Point(1, 1), Point(1, 0),
  Point(-1, -1), Point(0, -1), Point(-1, 0),
]
reference = Point(-1, -1)
n = len(points)
k = 2

k_nearest_points = [Point(-1, -1), Point(-1, 0)]
# or
k_nearest_points = [Point(-1, -1), Point(0, -1)]

Sort Colors

Given an array nums with n objects colored red, white, or blue, sort them in-place so that objects of the same color are adjacent, with the colors in the order red, white, and blue.

We will use the integers 0, 1, and 2 to represent the color red, white, and blue, respectively.

You must solve this problem without using the library's sort function

Example 1:

Input: nums = [2,0,2,1,1,0]
Output: [0,0,1,1,2,2]

Example 2:

Input: nums = [2,0,1]
Output: [0,1,2]

Constraints:

• n == nums.length
• 1 <= n <= 300
• nums[i] is either 0, 1, or 2.

Follow up: Could you come up with a one-pass algorithm using only constant extra space?

LeetCode

Generate Parentheses

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

Example 1:

Input: n = 3
Output: ["((()))","(()())","(())()","()(())","()()()"]

Example 2:

Input: n = 1
Output: ["()"]

Constraints:

• 1 <= n <= 8

LeetCode

Longest Valid Parentheses

Given a string containing just the characters '(' and ')', return the length of the longest valid (well-formed) parentheses substring.

Example 1:

Input: s = "(()"
Output: 2
Explanation: The longest valid parentheses substring is "()".

Example 2:

Input: s = ")()())"
Output: 4
Explanation: The longest valid parentheses substring is "()()".

Example 3:

Input: s = ""
Output: 0

Constraints:

• 0 <= s.length <= 3 * 10^4
• s[i] is '(', or ')'

LeetCode

Min By Column

Part 1 of 2

Imagine that we are working with a simple database. Each row associates column names (strings) with integer values. Here's a table with three rows:

a b c d
1 0 0 0
0 2 3 0
0 0 0 4

We might choose to represent a database table in JSON, as an array of objects. For example, the previous table could be written as:

[
  { "a": 1, "b": 0, "c": 0, "d": 0 },
  { "a": 0, "b": 2, "c": 3, "d": 0 },
  { "a": 0, "b": 0, "c": 0, "d": 4 }
]

Write a function, min_by_column, that takes a database table (as above), along with a column name, and returns the row that contains the minimum value for the given column. If a row doesn't have any value for the column, your function should behave as though the value for that column was zero.

Examples

table_1 = [
{"a": 1},
{"a": 2},
{"a": 3}
]
assert min_by_column(table_1, "a") == {"a": 1}

table_2 = [
{"a": 1, "b": 2},
{"a": 3, "b": 0}
]
assert min_by_column(table_2, "b") == {"a": 3, "b": 0}

table_3 = [
{"a": 1, "b": -2},
{"a": 3}
]
assert min_by_column(table_3, "b") == {"a": 1, "b": -2}

Part 2 of 2

In Part 1 you may have noticed that it's possible for two rows to be "tied", meaning that either would be an acceptable return value from min_by_column.

Consider:

table_4 = [
{"a": 1, "b": 2},
{"a": 1, "b": 3},
{"a": 1, "b": 4}
]
assert min_by_column(table_4, "a") == '???'

Since all three rows have the same value for column "a", all three rows are acceptable candidates to be returned by min_by_column(table, "a").

In these cases, it would be nice if users could specify additional columns (e.g. "b") to use as tie-breakers. A tie-breaker would only apply in cases where multiple rows share the same minimum value. In table_4 above, the row {"a": 1, "b": 2} is tied for the smallest "a" value (1) and of all the tied candidates, it has the smallest "b" value (2). If two records had equal values for "a" and also for "b" then another tie-breaker (e.g. "c") could be used. When records are tied with respect to all columns, any of the tied records may be considered the minimum.

Write a function min_by_columns that takes a database table and an ordered list of column names, and returns the row with the minimum column values using the tie-breaking logic above. Refactor min_by_column to use min_by_columns to produce its result.

Examples

table_5 = [
{"x": 1, "y": 3},
{"x": 1, "y": 0}
]
assert min_by_columns(table_5, ["x", "y"]) == {"x": 1, "y": 0}

table_6 = [
{"x": 2, "y": 3},
{"x": 2, "y": 1},
{"x": 1, "y": 10}
]
assert min_by_columns(table_6, ["x", "y"]) == {"x": 1, "y": 10}

table_7 = [
{"x": 3, "y": -1, "z": 0},
{"x": 1, "y": 10, "z": 1},
{"x": 1, "y": 10, "z": 0}
]
assert min_by_columns(table_7, ["x", "y", "z"]) == {"x": 1, "y": 10, "z": 0}

table_8 = [
{"x": 1, "y": 2, "z": 3},
{"x": 2, "y": 2, "z": 2}
]
assert min_by_columns(table_8, ["x", "y", "z"]) == {"x": 1, "y": 2, "z": 3}

Prefix Search

We have a database, and we'd like it to support these operations:

• insert(word): Inset a word to database
• look up(prefix): Return all the words starting with the given prefix
• delete(prefix): Delete all the words starting with the given prefix
• count(prefix)Count the number of words starting with the given prefix

Package Delivery

Part 1

Write a Delivery class (or object) that represents a delivery with a destination and distance. Deliveries require different sensors, depending on their distance.

Add a method, getNeededSensors, that returns a mapping of sensor name to the count of that sensor needed to complete the delivery according to these rules:

• If distance < 10 miles, require 1 gps and 1 temp sensor.
• If 10 <= distance < 100 miles require 1 gps, 2 temp, and 1 weight sensor.
• If distance >= 100 miles require 2 gps, 4 temp, and 2 weight sensors.

Part 2

Write a Scheduler class (or object) that represents a daily delivery scheduler with a set of available sensors. Add a method, scheduleDeliveries, that given a list of deliveries as an argument, returns a list of deliveries that can be made that day. Assume all deliveries will be leaving at the same time every day, so sensors can only be used once.

Test Cases to consider:

deliveryA = Delivery("A", 9)
deliveryB = Delivery("B", 15)
deliveryC = Delivery("C", 100)
scheduler = Scheduler({"gps": 2, "temp": 4, "weight": 2})
scheduler.scheduleDeliveries([deliveryA, deliveryB, deliveryC]) == [deliveryA, deliveryB]
scheduler.scheduleDeliveries([deliveryA, deliveryC, deliveryB]) == [deliveryA, deliveryB]
scheduler.scheduleDeliveries([deliveryC, deliveryA, deliveryB]) == [deliveryC]

Part 3

We get paid a flat fee for all deliveries. Modify the scheduleDeliveries function to maximize the number of deliveries that will be made in a day.

Our previous test:

scheduler.scheduleDeliveries([deliveryC, deliveryA, deliveryB]) ==[deliveryC]

Should now return:

scheduler.scheduleDeliveries([deliveryC, deliveryA, deliveryB]) ==[deliveryA, deliveryB]

Part 4

We recently purchased a new type of sensor, doorSensor. A doorSensor can be used in place of 1 weight sensor or in place of 2 temperature sensors at any time. Modify our existing functions to maximize the day's deliveries with the new sensor.

K-th biggest number

time complexity of retrieving the biggest number in a list: O(n) time complexity of retrieving the second-biggest number in a list: 2*O(n) = O(n) time complexity of retrieving the k-th biggest number in a list: if k is smaller than lg(n) we can retrieve the element in O(kn) and if k is bigger than lg(n) we can retrieve the element in O(nlg(n)) by sorting the list and returning the k-th element

Prefix and Suffix Search

LeetCode

Print N-bit binary numbers having more 1s than 0s

Given a positive integer n. Your task is to generate a string list of all n-bit binary numbers where, for any prefix of the number, there are more or an equal number of 1's than 0's. The numbers should be sorted in decreasing order of magnitude.

Example 1:

Input:
n = 2
Output:
"11, 10"
Explanation: Valid numbers are those where each prefix has more 1s than 0s:
11: all its prefixes (1 and 11) have more 1s than 0s.
10: all its prefixes (1 and 10) have more 1s than 0s.
So, the output is "11, 10".

Example 2:

Input:
n = 3
Output:
"111, 110, 101"
Explanation: Valid numbers are those where each prefix has more 1s than 0s.
111: all its prefixes (1, 11, and 111) have more 1s than 0s.
110: all its prefixes (1, 11, and 110) have more 1s than 0s.
101: all its prefixes (1, 10, and 101) have more 1s than 0s.
So, the output is "111, 110, 101".

User Task: Your task is to complete the function NBitBinary() which takes a single number as input n and returns the list of strings in decreasing order. You need not take any input or print anything.

class Solution:
  def NBitBinary(self, n):
    pass

Expected Time Complexity: O(|2n|) Expected Auxiliary Space: O(2n)

Constraints:

1 <= n <= 15

GeeksForGeeks

Kadane's Algorithm

Given an array Arr[] of N integers. Find the contiguous sub-array (containing at least one number) which has the maximum sum and return its sum.

Example 1:

Input:
N = 5
Arr[] = {1,2,3,-2,5}
Output:
9
Explanation:
Max subarray sum is 9
of elements (1, 2, 3, -2, 5) which
is a contiguous subarray.

Example 2:

Input:
N = 4
Arr[] = {-1,-2,-3,-4}
Output:
-1
Explanation:
Max subarray sum is -1
of element (-1)

Your Task: You don't need to read input or print anything. The task is to complete the function maxSubarraySum() which takes Arr[] and N as input parameters and returns the sum of subarray with maximum sum.

Expected Time Complexity: O(N) Expected Auxiliary Space: O(1)

Constraints:

1 ≤ N ≤ 10^6
-10^7 ≤ A[i] ≤ 10^7

GeeksForGeeks

Find All Duplicates in an Array

Given an integer array nums of length n where all the integers of nums are in the range [1, n] and each integer appears once or twice, return an array of all the integers that appears twice.

You must write an algorithm that runs in O(n) time and uses only constant extra space.

Example 1:

Input: nums = [4,3,2,7,8,2,3,1]
Output: [2,3]

Example 2:

Input: nums = [1,1,2]
Output: [1]

Example 3:

Input: nums = [1]
Output: []

Constraints:

• n == nums.length
• 1 <= n <= 105
• 1 <= nums[i] <= n

Each element in nums appears once or twice.

LeetCode

First Missing Positive

Given an unsorted integer array nums. Return the smallest positive integer that is not present in nums.

You must implement an algorithm that runs in O(n) time and uses O(1) auxiliary space.

Example 1:

Input: nums = [1,2,0]
Output: 3
Explanation: The numbers in the range [1,2] are all in the array.

Example 2:

Input: nums = [3,4,-1,1]
Output: 2
Explanation: 1 is in the array but 2 is missing.

Example 3:

Input: nums = [7,8,9,11,12]
Output: 1
Explanation: The smallest positive integer 1 is missing.

Constraints:

• 1 <= nums.length <= 105
• 231 <= nums[i] <= 231 - 1

LeetCode

Excel Sheet Column Title

Given an integer columnNumber, return its corresponding column title as it appears in an Excel sheet.

For example:

A -> 1
B -> 2
C -> 3
...
Z -> 26
AA -> 27
AB -> 28
...

Example 1:

Input: columnNumber = 1
Output: "A"

Example 2:

Input: columnNumber = 28
Output: "AB"

Example 3:

Input: columnNumber = 701
Output: "ZY"

Constraints:

• 1 <= columnNumber <= 231 - 1

LeetCode

Permutation Sequence

The set [1, 2, 3, ..., n] contains a total of n! unique permutations.

By listing and labeling all the permutations in order, we get the following sequence for n = 3:

"123"
"132"
"213"
"231"
"312"
"321"

Given n and k, return the kth permutation sequence.

Example 1:

Input: n = 3, k = 3
Output: "213"

Example 2:

Input: n = 4, k = 9
Output: "2314"

Example 3:

Input: n = 3, k = 1
Output: "123"

Constraints:

• 1 <= n <= 9
• 1 <= k <= n!

Length of Longest Subarray With at Most K Frequency

You are given an integer array nums and an integer k. The frequency of an element x is the number of times it occurs in an array. An array is called good if the frequency of each element in this array is less than or equal to k. Return the length of the longest good subarray of nums. A subarray is a contiguous non-empty sequence of elements within an array.

Example 1:

Input: nums = [1,2,3,1,2,3,1,2], k = 2
Output: 6
Explanation: The longest possible good subarray is [1,2,3,1,2,3] since the values 1, 2, and 3 occur at most twice in this subarray. Note that the subarrays [2,3,1,2,3,1] and [3,1,2,3,1,2] are also good.
It can be shown that there are no good subarrays with length more than 6.

Example 2:

Input: nums = [1,2,1,2,1,2,1,2], k = 1
Output: 2
Explanation: The longest possible good subarray is [1,2] since the values 1 and 2 occur at most once in this subarray. Note that the subarray [2,1] is also good.
It can be shown that there are no good subarrays with length more than 2.

Example 3:

Input: nums = [5,5,5,5,5,5,5], k = 4
Output: 4
Explanation: The longest possible good subarray is [5,5,5,5] since the value 5 occurs 4 times in this subarray.
It can be shown that there are no good subarrays with length more than 4.

Constraints:

• 1 <= nums.length <= 10^5
• 1 <= nums[i] <= 10^9
• 1 <= k <= nums.length

Count Subarrays Where Max Element Appears at Least K Times

You are given an integer array nums and a positive integer k. Return the number of subarrays where the maximum element of nums appears at least k times in that subarray. A subarray is a contiguous sequence of elements within an array.

Example 1:

Input: nums = [1,3,2,3,3], k = 2
Output: 6
Explanation: The subarrays that contain the element 3 at least 2 times are: [1,3,2,3], [1,3,2,3,3], [3,2,3], [3,2,3,3], [2,3,3] and [3,3].

Example 2:

Input: nums = [1,4,2,1], k = 3
Output: 0
Explanation: No subarray contains the element 4 at least 3 times.

Constraints:

• 1 <= nums.length <= 10^5
• 1 <= nums[i] <= 10^6
• 1 <= k <= 105