algodeck

Sort

Bubble sort algorithm

Walk through a collection and compares 2 elements at a time

If they are out of order, swap them

Continue until the entire collection is sorted

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Bubble sort complexity and stability

Time: O(n²)

Space: O(1)

Stable

#complexity #sort

Counting sort complexity, stability, use case

Time complexity: O(n + k) // n is the number of elements, k is the range (the maximum element)

Space complexity: O(k)

Stable

Use case: known and small range of possible integers

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Counting sort algorithm

If range r is known

1) Create an array of size r where each a[i] represents the number of occurences of i

2) Modify the array to store the cumulative sum (if a=[1, 3, 0, 2] => [1, 4, 4, 6])

3) Right shift the array with a backward iteration (element at index 0 is 0 => [0, 1, 4, 4]) Now a[i] represents the first index of i if array was sorted

4) Create the sorted array by filling the elements from their first index

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Heapsort algorithm

• Build a max heap from the array
• For i from n-1 to 0:
  1. Swap the largest element (at index 0) with i
  2. Heapify the remaining elements (0.. i -1) by putting the root element at its correct position (keep swapping element with biggest child until there is a max heap violation on a node)

#heap #sort

Heapsort complexity, stability, use case

Time: Theta(n log n)

Space: O(1)

Unstable

Use case: space constrained environment with O(n log n) time guarantee

Yet, not stable and not cache friendly

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Insertion sort algorithm

From i to 0..n, insert a[i] to its correct position to the left (0..i)

Used by humans

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Insertion sort complexity, stability, use case

Time: O(n²)

Space: O(1)

Stable

Use case: partially sorted structure

#complexity #sort

Mergesort algorithm

Splits a collection into 2 halves, sort the 2 halves (recursive call) then merge them together to form one sorted collection

void mergeSort(int[] a) {
	int[] helper = new int[a.length];
	mergeSort(a, helper, 0, a.length - 1);
}

void mergeSort(int a[], int helper[], int lo, int hi) {
	if (lo < hi) {
		int mid = (lo + hi) / 2;

		mergeSort(a, helper, lo, mid);
		mergeSort(a, helper, mid + 1, hi);
		merge(a, helper, lo, mid, hi);
	}
}

private void merge(int[] a, int[] helper, int lo, int mid, int hi) {
	// Copy into helper
	for (int i = lo; i <= hi; i++) {
		helper[i] = a[i];
	}

	int p1 = lo; // Pointer on the first half
	int p2 = mid + 1; // Pointer on the second half
	int index = lo; // Index of a

	// Copy the smallest values from either the left or the right side back to the original array
	while (p1 <= mid && p2 <= hi) {
		if (helper[p1] <= helper[p2]) {
			a[index] = helper[p1];
			p1++;
		} else {
			a[index] = helper[p2];
			p2++;
		}
		index++;
	}

	// Copy the eventual rest of the left side of the array into the target array
	while (p1 <= mid) {
		a[index] = helper[p1];
		index++;
		p1++;
	}
}

Further Reading

• Making Sense of Merge Sort - Part 1 by Vaidehi Joshi
• Making Sense of Merge Sort - Part 2 by Vaidehi Joshi

#sort

Mergesort complexity, stability, use case

Time: Theta(n log n)

Space: O(n)

Stable

Use case: good worst case time complexity and stable, good with linked list

#complexity #sort

Quicksort algorithm

Sort a collection by repeatedly choosing a pivot and partitioning the collection around it (smaller before, larger after)

Here the pivot will be the last element of the subarray

In an ideal world, the pivot would be the middle element so that we partition the array in two subsets of equal size

The worst case is to find a pivot element at the top left or top right index of the subarray

void quickSort(int[] a) {
	quickSort(a, 0, a.length - 1);
}

void quickSort(int a[], int lo, int hi) {
	if (lo < hi) {
		int pivot = partition(a, lo, hi);
		quickSort(a, lo, pivot - 1);
		quickSort(a, pivot + 1, hi);
	}
}

// Returns an index so that all element before that index are smaller
// And all element after are bigger
int partition(int a[], int lo, int hi) {
	int pivot = a[hi];
	int pivotIndex = lo; // Will represent the pivot index

	// Iterate using the two pointers technique
	for (int i = lo; i < hi; i++) {
		// If the current index is smaller, swap and increment pivot index
		if (a[i] <= pivot) {
			swap(a, pivotIndex++, i);
		}
	}

	swap(a, pivotIndex, hi);
	return pivotIndex;
}

#sort

Quicksort complexity, stability, use case

Time: best and average O(n log n), worst O(n²) if the array is already sorted in ascending or descending order

Space: O(log n) // In-place sorting algorithm

Not stable

Use case: in practice, quicksort is often faster than merge sort due to better locality (not applicable with linked list so in this case we prefer mergesort)

#complexity #sort

Radix sort algorithm

Sort by applying counting sort on one digit at a time (least to most significant) Each new level must be stable (if equals, keep the order of the previous level)

Example:

• 53, 89, 150, 36, 633, 233
• Counting sort on digit 0 => 150, 53, 633, 36, 89
• Counting sort on digit 1 => 633, 233, 36, 150, 53, 89
• Counting sort on digit 2 => 36, 53, 89, 150, 233, 633 // If does not exist (like 36) it is replaced by 0

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Radix sort complexity, stability, use case

Time complexity: O(nk) // n is the number of elements, k is the maximum number of digits for a number

Space complexity: O(k)

Stable

Use case: if k < log(n) (for example 1M of elements from 0..1000 as 4 < log(1M))

#complexity #sort

Selection sort algorithm

From i to 0..n, find repeatedly the min element then swap it with i

#sort

Selection sort complexity

Time: Theta(n²)

Space: O(1)

#complexity #sort

Shuffling an array

Fisher-Yates shuffle algorithm:

• Iterate over each element (i)
• Pick a random index (from 0 to i included) and swap with the current element

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