Questions tagged [algorithm]
An algorithm is a sequence of well-defined steps that defines an abstract solution to a problem. Use this tag when your issue is related to algorithm design.
algorithm
13,354
questions
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How do I check if an array includes a value in JavaScript?
What is the most concise and efficient way to find out if a JavaScript array contains a value?
This is the only way I know to do it:
function contains(a, obj) {
for (var i = 0; i < a.length; i++...
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5373
votes
43
answers
831k
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What is a plain English explanation of "Big O" notation?
I'd prefer as little formal definition as possible and simple mathematics.
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1676
votes
22
answers
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What is the best algorithm for overriding GetHashCode?
In .NET, the GetHashCode method is used in a lot of places throughout the .NET base class libraries. Implementing it properly is especially important to find items quickly in a collection or when ...
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978
votes
24
answers
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Big O, how do you calculate/approximate it?
Most people with a degree in CS will certainly know what Big O stands for.
It helps us to measure how well an algorithm scales.
But I'm curious, how do you calculate or approximate the complexity of ...
- 18.3k
1468
votes
58
answers
2.2m
views
Removing duplicates in lists
How can I check if a list has any duplicates and return a new list without duplicates?
- 20.5k
1039
votes
10
answers
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views
How can I find the time complexity of an algorithm?
I have gone through Google and Stack Overflow search, but nowhere I was able to find a clear and straightforward explanation for how to calculate time complexity.
What do I know already?
Say for code ...
- 47.4k
853
votes
41
answers
1.1m
views
How do I generate all permutations of a list?
How do I generate all the permutations of a list? For example:
permutations([])
[]
permutations([1])
[1]
permutations([1, 2])
[1, 2]
[2, 1]
permutations([1, 2, 3])
[1, 2, 3]
[1, 3, 2]
[2, 1, 3]
[2, ...
- 13.5k
1013
votes
66
answers
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views
Count the number of set bits in a 32-bit integer
8 bits representing the number 7 look like this:
00000111
Three bits are set.
What are the algorithms to determine the number of set bits in a 32-bit integer?
1160
votes
49
answers
1.2m
views
Calculate distance between two latitude-longitude points? (Haversine formula)
How do I calculate the distance between two points specified by latitude and longitude?
For clarification, I'd like the distance in kilometers; the points use the WGS84 system and I'd like to ...
- 15.2k
661
votes
32
answers
651k
views
How do you compare float and double while accounting for precision loss?
What would be the most efficient way to compare two double or two float values?
Simply doing this is not correct:
bool CompareDoubles1 (double A, double B)
{
return A == B;
}
But something like:
...
- 6,651
645
votes
77
answers
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views
Algorithm to return all combinations of k elements from n
I want to write a function that takes an array of letters as an argument and a number of those letters to select.
Say you provide an array of 8 letters and want to select 3 letters from that. Then ...
576
votes
15
answers
140k
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What is the most efficient/elegant way to parse a flat table into a tree?
Assume you have a flat table that stores an ordered tree hierarchy:
Id Name ParentId Order
1 'Node 1' 0 10
2 'Node 1.1' 1 10
3 'Node 2' 0 ...
- 335k
217
votes
30
answers
187k
views
Rolling or sliding window iterator?
I need a rolling window (aka sliding window) iterable over a sequence/iterator/generator. (Default Python iteration could be considered a special case, where the window length is 1.) I'm currently ...
- 5,770
179
votes
36
answers
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Cartesian product of multiple arrays in JavaScript
How would you implement the Cartesian product of multiple arrays in JavaScript?
As an example,
cartesian([1, 2], [10, 20], [100, 200, 300])
should return
[
[1, 10, 100],
[1, 10, 200],
[1, ...
- 20k
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votes
29
answers
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views
What is tail recursion?
Whilst starting to learn lisp, I've come across the term tail-recursive. What does it mean exactly?
311
votes
32
answers
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Finding all possible combinations of numbers to reach a given sum
How would you go about testing all possible combinations of additions from a given set N of numbers so they add up to a given final number?
A brief example:
Set of numbers to add: N = {1,5,22,15,0,.....
- 19.5k
1103
votes
10
answers
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views
What is tail call optimization?
Very simply, what is tail-call optimization?
More specifically, what are some small code snippets where it could be applied, and where not, with an explanation of why?
- 13.2k
197
votes
23
answers
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views
Unique (non-repeating) random numbers in O(1)?
I'd like to generate unique random numbers between 0 and 1000 that never repeat (i.e. 6 doesn't show up twice), but that doesn't resort to something like an O(N) search of previous values to do it. Is ...
- 46k
236
votes
20
answers
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Understanding recursion [closed]
I'm having major trouble understanding recursion at school. Whenever the professor is talking about it, I seem to get it but as soon as I try it on my own it completely blows my brains.
I was trying ...
- 2,393
462
votes
57
answers
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Generating all permutations of a given string
What is an elegant way to find all the permutations of a string. E.g. permutation for ba, would be ba and ab, but what about longer string such as abcdefgh? Is there any Java implementation example?
- 66.2k
312
votes
20
answers
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The most efficient way to implement an integer based power function pow(int, int)
What is the most efficient way given to raise an integer to the power of another integer in C?
// 2^3
pow(2,3) == 8
// 5^5
pow(5,5) == 3125
- 64.9k
217
votes
18
answers
258k
views
Fastest way to flatten / un-flatten nested JavaScript objects
I threw some code together to flatten and un-flatten complex/nested JavaScript objects. It works, but it's a bit slow (triggers the 'long script' warning).
For the flattened names I want "." ...
- 21k
574
votes
53
answers
1.0m
views
Best way to reverse a string
I've just had to write a string reverse function in C# 2.0 (i.e. LINQ not available) and came up with this:
public string Reverse(string text)
{
char[] cArray = text.ToCharArray();
string ...
- 66.1k
179
votes
13
answers
55k
views
Is using Random and OrderBy a good shuffle algorithm? [closed]
I have read an article about various shuffle algorithms over at Coding Horror. I have seen that somewhere people have done this to shuffle a list:
var r = new Random();
var shuffled = ordered.OrderBy(...
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347
votes
2
answers
36k
views
How to implement classic sorting algorithms in modern C++?
The std::sort algorithm (and its cousins std::partial_sort and std::nth_element) from the C++ Standard Library is in most implementations a complicated and hybrid amalgamation of more elementary ...
- 69.9k
447
votes
12
answers
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Image comparison - fast algorithm
I'm looking to create a base table of images and then compare any new images against that to determine if the new image is an exact (or close) duplicate of the base.
For example: if you want to ...
- 23k
341
votes
25
answers
374k
views
How to determine if a point is in a 2D triangle? [closed]
What is the simplest algorithm to determine if a point is inside a 2d triangle?
- 3,493
558
votes
13
answers
230k
views
Why do we check up to the square root of a number to determine if the number is prime?
To test whether a number is prime or not, why do we have to test whether it is divisible only up to the square root of that number?
- 6,525
235
votes
32
answers
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views
How to find the kth largest element in an unsorted array of length n in O(n)?
I believe there's a way to find the kth largest element in an unsorted array of length n in O(n). Or perhaps it's "expected" O(n) or something. How can we do this?
- 44k
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votes
32
answers
1.6m
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What does O(log n) mean exactly?
I am learning about Big O Notation running times and amortized times. I understand the notion of O(n) linear time, meaning that the size of the input affects the growth of the algorithm ...
- 107k
191
votes
21
answers
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views
Best way to randomize an array with .NET
What is the best way to randomize an array of strings with .NET? My array contains about 500 strings and I'd like to create a new Array with the same strings but in a random order.
Please include a C#...
- 15.1k
161
votes
30
answers
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views
How to create the most compact mapping n → isprime(n) up to a limit N?
Naturally, for bool isprime(number) there would be a data structure I could query.
I define the best algorithm, to be the algorithm that produces a data structure with lowest memory consumption for ...
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251
votes
24
answers
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views
HSL to RGB color conversion
I am looking for an algorithm to convert between HSL color to RGB.
It seems to me that HSL is not very widely used so I am not having much luck searching for a converter.
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164
votes
34
answers
209k
views
What is the fastest/most efficient way to find the highest set bit (msb) in an integer in C?
If I have some integer n, and I want to know the position of the most significant bit (that is, if the least significant bit is on the right, I want to know the position of the farthest left bit that ...
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18
votes
3
answers
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Fast bignum square computation
To speed up my bignum divisons I need to speed up operation y = x^2 for bigints which are represented as dynamic arrays of unsigned DWORDs. To be clear:
DWORD x[n+1] = { LSW, ......, MSW };
where n+1 ...
526
votes
8
answers
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views
What is Constant Amortized Time?
What is meant by "Constant Amortized Time" when talking about time complexity of an algorithm?
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votes
29
answers
230k
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Listing all permutations of a string/integer
A common task in programming interviews (not from my experience of interviews though) is to take a string or an integer and list every possible permutation.
Is there an example of how this is done ...
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298
votes
10
answers
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How to sort in-place using the merge sort algorithm?
I know the question is not too specific.
All I want is someone to tell me how to convert a normal merge sort into an in-place merge sort (or a merge sort with constant extra space overhead).
All I ...
- 92.4k
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votes
13
answers
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Why does Java's hashCode() in String use 31 as a multiplier?
Per the Java documentation, the hash code for a String object is computed as:
s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]
using int arithmetic, where s[i] is the
ith character of the string,...
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votes
31
answers
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views
How to check if a number is a power of 2
Today I needed a simple algorithm for checking if a number is a power of 2.
The algorithm needs to be:
Simple
Correct for any ulong value.
I came up with this simple algorithm:
private bool ...
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450
votes
10
answers
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views
What is stability in sorting algorithms and why is it important?
I'm very curious, why stability is or is not important in sorting algorithms?
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244
votes
29
answers
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Detecting endianness programmatically in a C++ program
Is there a programmatic way to detect whether or not you are on a big-endian or little-endian architecture? I need to be able to write code that will execute on an Intel or PPC system and use exactly ...
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votes
14
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Which cycle detection within a directed graph are more efficient than O(n^2)? [closed]
Is there an algorithm that is more time efficient than O(n^2) for detecting cycles within a directed graph?
I have a directed graph representing a schedule of jobs that need to be executed, a job ...
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votes
19
answers
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How can building a heap be O(n) time complexity?
Can someone help explain how can building a heap be O(n) complexity?
Inserting an item into a heap is O(log n), and the insert is repeated n/2 times (the remainder are leaves, and can't violate the ...
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votes
64
answers
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How do you rotate a two dimensional array?
Inspired by Raymond Chen's post, say you have a 4x4 two dimensional array, write a function that rotates it 90 degrees. Raymond links to a solution in pseudo code, but I'd like to see some real world ...
88
votes
25
answers
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views
Sort on a string that may contain a number
I need to write a Java Comparator class that compares Strings, however with one twist. If the two strings it is comparing are the same at the beginning and end of the string are the same, and the ...
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130
votes
38
answers
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Counting inversions in an array
I'm designing an algorithm to do the following: Given array A[1... n], for every i < j, find all inversion pairs such that A[i] > A[j]. I'm using merge sort and copying array A to array B and ...
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votes
28
answers
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Efficient Algorithm for Bit Reversal (from MSB->LSB to LSB->MSB) in C
What is the most efficient algorithm to achieve the following:
0010 0000 => 0000 0100
The conversion is from MSB->LSB to LSB->MSB. All bits must be reversed; that is, this is not endianness-...
- 3,007
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votes
32
answers
628k
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Check if all elements in a list are identical
I need a function which takes in a list and outputs True if all elements in the input list evaluate as equal to each other using the standard equality operator and False otherwise.
I feel it would be ...
- 50.7k
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votes
21
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Determine if two rectangles overlap each other?
I am trying to write a C++ program that takes the following inputs from the user to construct rectangles (between 2 and 5): height, width, x-pos, y-pos. All of these rectangles will exist parallel to ...
- 5,435