algorithms – Do the following two CFGs describe the same language

No. The word “abab” is described by (1) but not (2). You can test and visualize this here.

Trying to think about it inductively: (1) can only prepend “a” or “b” to a string while (2) can only prepend “a” and only append “b”.

Switching the order of a production does not necessarily change the language the grammar represents, but it is in this case. As to the second part of your question, there is no generalizable or efficient way to determine whether two CFGs describe the same language.

algorithms – Time complexity of finding median in data stream

I was reading a solution to the problem in the title on leetcode and the article says that the time complexity of the following solution is O(n)

  1. setup a data structure to hold stream value and insert new element in the right place using linear search or binary search
  2. return median

my solution is as follows:

class MedianFinder:

    def __init__(self):
        initialize your data structure here.
        self.arr = ()

    def addNum(self, num: int) -> None:
        idx = bisect.bisect_left(self.arr, num)
        self.arr.insert(idx, num)

    def findMedian(self) -> float:
        # self.arr.sort()
        if len(self.arr) % 2 != 0:
            return self.arr(len(self.arr)//2)
            return (self.arr(len(self.arr)//2 -1) + self.arr(len(self.arr)//2 ))/2

My question is about the time complexity of the push method. the binary search will take O(log n) to find the index. the insert will take O(n). but since the method will be called on the stream, will the complexity be O(n^2) or O(n) ?

enter image description here

algorithms – How could a bank software intend to transfer $82.56 but result in a error transfer of $1,205,619.56

algorithms – How could a bank software intend to transfer $82.56 but result in a error transfer of $1,205,619.56 – Software Engineering Stack Exchange

algorithms – “Let $Pi$ and $Pi$’ be two NP-complete problems, prove or deny $Pi ’propto_{poly}Pi$”

I am facing a proving question.

“Let $Pi$ and $Pi$’ be two NP-complete problems, prove or deny $Pi’propto_{poly}Pi$

I do not that understand the meaning of this question. Isn’t it a self-evident truth to be $Pi ’propto_{poly}Pi$ if both of them are NP-complete problems?

algorithms – Pruning: Outlier Candidate Selection

I am using method 3.3 described in Outlier Detection using Isolation Forest and Local Outlier

It states:

Specify a dataset: $𝐷={𝑑_1, 𝑑_2, …, 𝑑_𝑛}$.
Here, 𝑛 is the sample number of 𝐷. $𝑑_𝑖$ is an attribute in $𝐷$, and $𝑑_𝑖={𝑥_1,
𝑥_2, …, 𝑥_𝑛}.$
is a certain data value of the attribute $𝑑_𝑖$
The outlier coefficient of the attribute is defined as:
enter image description here

Here, $bar{𝑥}$ is the mean of the attribute $𝑑_𝑖$ and $𝑓𝑑_𝑖$
is used to
measure the degree of dispersion of the attribute $𝑑_𝑖$
. Calculate
the outlier coefficient of each attribute in the dataset, and
get the outlier coefficient vector $𝐷_𝑓$ of the dataset, which is
recorded as:
$𝐷_𝑓 = 𝑓𝑑_1, 𝑓𝑑_2, …, 𝑓𝑑_n$

However, I do not understand what value to select for $x_j$. What does a ‘certain data value’ mean in this case?

algorithms – Is this radix sort implementation in-place or out-place?

Below is the implementation of Radix Sort in python taken from here

def countingSort(arr, exp1):
n = len(arr)
# The output array elements that will have sorted arr
output = (0) * (n)
# initialize count array as 0
count = (0) * (10)
# Store count of occurrences in count()
for i in range(0, n):
    index = (arr(i) / exp1)
    count(int(index % 10)) += 1
# Change count(i) so that count(i) now contains actual
# position of this digit in output array
for i in range(1, 10):
    count(i) += count(i - 1)
# Build the output array
i = n - 1
while i >= 0:
    index = (arr(i) / exp1)
    output(count(int(index % 10)) - 1) = arr(i)
    count(int(index % 10)) -= 1
    i -= 1
# Copying the output array to arr(),
# so that arr now contains sorted numbers
i = 0
for i in range(0, len(arr)):
    arr(i) = output(i)

def radixSort(arr):
# Find the maximum number to know number of digits
max1 = max(arr)
# Do counting sort for every digit. Note that instead
# of passing digit number, exp is passed. exp is 10^i
# where i is current digit number
exp = 1
while max1 / exp > 0:
    countingSort(arr, exp)
    exp *= 10
# This code is contributed by Mohit Kumra
# Edited by Patrick Gallagher

Generally, we can implement algorithms in-place and out-of-place. However, I can’t tell whether the above implementation is out-of-place.

algorithms – Check if graph is regular given adjacency matrix

I have the following problem:

Write a function
whose input is an adjacency matrix A of a graph G. The function returns true
if G is a regular graph and false otherwise.

I understand that a graph is regular if the degrees of all the vertices are the same.

Therefore, I have written the following code which calculates the amount of degrees for each column:

def sumColumns(A):
  columns = ()
  for i in range(0, len(A(0))):
    total = 0
    for j in range(0, len(A)):
      total += A(j)(i)
    if total > 0:
  return columns

def isRegularGraph(A):

  # Get list of degrees
  cd = sumColumns(A)

  if not len(cd) > 0 or  cd(0) != cd(len(cd) - 1):
    return False
  # Do comparisons from i to i - 1
  for i in range(0, len(cd) - 1):
    if cd(i) != cd(i + 1):
      return False
  return True

My question is, do I also need to check row wise if the number of degrees are the same column wise?

Is there a better algorithm to do this?

algorithms – Finding all pairs of points with no point in between

Suppose there are $n$ points $p_1,p_2,dots,p_n$ with color red or blue on a line. We want to find all pairs $(p_i,p_j)$ whose color is distinct and such that there are no points between them. If there are $k$ pairs with the described property, design an algorithm with $O(nlog k)$ that uses idea of divide and pruning.

I think if we check all points we can solve this problem, but running time will exceed $O(nlog k)$.

I think for solving this problem we can use projective geometry duality, but I am stuck. Any help would be appreciated.

algorithms – Finding distinct pairs in geometric settings

Suppose there are $n$ points $p_1,p_2,…,p_n$ with color red or blue on a line. We want to find all pairs $(p_i,p_j)$ their color is distinct and there are no points between theme. If there are $k$ pairs with described design an algorithm with $O(nlog k)$ that use idea divide and pruning.

I think if we check all point we can solve this problem , but running time is not $O(nlog k)$.

I think for solving this problem we can use the duality that present in this page

But i get stuck to solve my problem. any help be appreciated.

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