
#### Problem Statement

Given an array of n elements, write a function to sort the array

#### Approach

- Find a mid point and divide the array into to halves based on the mid point
- Recursively call the merge sort function for both the halves
- Merge the two sorted halves to get the sorted array

#### Time Complexity

`O(n log n)`

#### Space Complexity

`O(n)`

#### Example

```
arr = [1, 3, 9, 5, 0, 2]  

Divide the array in two halves [1, 3, 9] and [5, 0, 2]

Recursively call merge sort function for both these halves which will provide sorted halves
=> [1, 3, 9] & [0, 2, 5]

Now merge both these halves to get the sorted array [0, 1, 2, 3, 5, 9]
```

#### Code Implementation Links

- [Java](https://github.com/TheAlgorithms/Java/blob/master/Sorts/MergeSort.java)
- [C++](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/sorting/merge_sort.cpp)
- [Python](https://github.com/TheAlgorithms/Python/blob/master/sorts/merge_sort.py)
- [C-Sharp](https://github.com/TheAlgorithms/C-Sharp/blob/master/Algorithms/Sorters/Comparison/MergeSorter.cs)
- [C](https://github.com/TheAlgorithms/C/blob/master/sorting/merge_sort.c)
- [Ruby](https://github.com/TheAlgorithms/Ruby/blob/master/sorting/merge_sort.rb)

#### Video Explanation

[A CS50 video explaining the Merge Sort Algorithm](https://www.youtube.com/watch?v=EeQ8pwjQxTM)


Mergesort

Sha1

Bead Sort

Bogo Sort


#### Problem Statement

Given an unsorted array of n elements, write a function to sort the array

#### Approach

- select the first element of the array
- compare it with its next element
- if it is larger than the next element then swap them
- else do nothing
- keep doing this for every index of the array
- repeat the above process n times.

#### Time Complexity

`O(n^2)` Worst case performance

`O(n)` Best-case performance

`O(n^2)` Average performance

#### Space Complexity

`O(1)` Worst case

#### Founder's Name

- The term âBubble Sortâ was first used by Iverson, K in 1962.

#### Example

```
arr[] = {10, 80, 40, 30}
Indexes: 0   1   2   3    

1. Index = 0, Number = 10
2. 10 < 80, do nothing and continue

3. Index = 1, Number = 80
4. 80 > 40, swap 80 and 40
5. The array now is {10, 40, 80, 30}

6. Index = 2, Number = 80
7. 80 > 30, swap 80 and 30
8. The array now is {10, 40, 30, 80}

Repeat the Above Steps again

arr[] = {10, 40, 30, 80}
Indexes: 0   1   2   3   

1. Index = 0, Number = 10
2. 10 < 40, do nothing and continue

3. Index = 1, Number = 40
4. 40 > 30, swap 40 and 30
5. The array now is {10, 30, 40, 80}

6. Index = 2, Number = 40
7. 40 < 80, do nothing
8. The array now is {10, 30, 40, 80}

Repeat the Above Steps again

arr[] = {10, 30, 40, 80}
Indexes: 0   1   2   3   

1. Index = 0, Number = 10
2. 10 < 30, do nothing and continue

3. Index = 1, Number = 30
4. 30 < 40, do nothing and continue

5. Index = 2, Number = 40
6. 40 < 80, do nothing

Since there are no swaps in above steps, it means the array is sorted and we can stop here.
```

#### Code Implementation Links

- [Java](https://github.com/TheAlgorithms/Java/blob/master/Sorts/BubbleSort.java)
- [C++](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/sorting/bubble_sort.cpp)
- [Python](https://github.com/TheAlgorithms/Python/blob/master/sorts/bubble_sort.py)
- [C-Sharp](https://github.com/TheAlgorithms/C-Sharp/blob/master/Algorithms/Sorters/Comparison/BubbleSorter.cs)
- [Go](https://github.com/TheAlgorithms/Go/blob/master/sorts/bubblesort.go)
- [Ruby](https://github.com/TheAlgorithms/Ruby/blob/master/sorting/bubble_sort.rb)
- [C](https://github.com/TheAlgorithms/C/blob/master/sorting/bubble_sort.c)
- [Scala](https://github.com/TheAlgorithms/Scala/blob/master/src/main/scala/Sort/BubbleSort.scala)
- [Javascript](https://github.com/TheAlgorithms/Javascript/blob/master/Sorts/BubbleSort.js)

#### Video Explanation

[A video explaining the Bubble Sort Algorithm](https://www.youtube.com/watch?v=Jdtq5uKz-w4)

#### Others

Bubble sort is also known as Sinking sort.

#### Animation Explanation

- [Tute Board](https://boardhub.github.io/tute/?wd=bubbleSortAlgo2)


Bubble Sort

Comb Sort

Counting Sort

Gnome Sort


#### Problem Statement

Given an unsorted array of n elements, write a function to sort the array

#### Approach

- Build a max heap from the input data.
- At this point, the largest item is stored at the root of the heap. Replace it with the last item of the heap followed by reducing the size of heap by 1. Finally, heapify the root of tree.
- Repeat above steps while size of heap is greater than 1.

#### Time Complexity

`O(n log n)` Worst case performance

`O(n log n)` (distinct keys)
or O(n) (equal keys) Best-case performance

`O(n log n)` Average performance

#### Space Complexity

`O(1)` Worst case auxiliary


#### Example
 ```
Input data: 4, 10, 3, 5, 1
         4(0)
        /   \
     10(1)   3(2)
    /   \
 5(3)    1(4)

The numbers in bracket represent the indices in the array
representation of data.

Applying heapify procedure to index 1:
         4(0)
        /   \
    10(1)    3(2)
    /   \
5(3)    1(4)

Applying heapify procedure to index 0:
        10(0)
        /  \
     5(1)  3(2)
    /   \
 4(3)    1(4)
The heapify procedure calls itself recursively to build heap
 in top down manner.
  ```

![heap-image](https://upload.wikimedia.org/wikipedia/commons/1/1b/Sorting_heapsort_anim.gif "Heap Sort")

#### Code Implementation Links

- [Java](https://github.com/TheAlgorithms/Java/blob/master/Sorts/HeapSort.java)
- [C++](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/sorting/heap_sort.cpp)
- [Python](https://github.com/TheAlgorithms/Python/blob/master/sorts/heap_sort.py)
- [Go](https://github.com/TheAlgorithms/Go/blob/master/sorts/heapsort.go)
- [Ruby](https://github.com/TheAlgorithms/Ruby/blob/master/sorting/heap_sort.rb)
- [C-sharp](https://github.com/TheAlgorithms/C-Sharp/blob/master/Algorithms/Sorters/Comparison/HeapSorter.cs)
- [C](https://github.com/TheAlgorithms/C/blob/master/sorting/heap_sort.c)
- [Javascript](https://github.com/TheAlgorithms/Javascript/blob/master/Sorts/HeapSort.js)

#### Video Explanation

[A video explaining the Selection Sort Algorithm](https://www.youtube.com/watch?v=MtQL_ll5KhQ)


Heap Sort


#### Problem Statement

Given an array of n elements, write a function to sort the array in increasing order.

#### Approach

- Define a "key" index, the subarray to the left of which is sorted.
- Initiate "key" as 1, ie. the second element of array(as there is only one element to left of the second element, which can be considered as sorted array with one element).
- If value of element at (key - 1) position is less than value of element at (key) position; increment "key".
- Else move elements of sorted subarray that are greater than value of element at "key" to one position ahead of their current position. Put the value of element at "key" in the newly created void.

#### Time Complexity

- `Ð(n^2)` comparisons, `Ð(n^2)` swaps -- Worst Case

- `O(n)` comparisons, `O(1)` swaps -- Best Case

#### Space Complexity

`O(1)` -- (No extra space needed, sorting done in place)

#### Example

```

12, 11, 13, 5, 6

Let us loop for i = 1 (second element of the array) to 4 (Size of input array)

i = 1.
Since 11 is smaller than 12, move 12 and insert 11 before 12
11, 12, 13, 5, 6

i = 2.
13 will remain at its position as all elements in sorted subarray are smaller than 13
11, 12, 13, 5, 6

i = 3.
5 will move to the beginning,
and all other elements from 11 to 13 will move one position ahead of their current position.
5, 11, 12, 13, 6

i = 4.
6 will move to position after 5,
and elements from 11 to 13 will move one position ahead of their current position.
5, 6, 11, 12, 13  -- sorted array
```

#### Code Implementation Links

- [Java](https://github.com/TheAlgorithms/Java/blob/master/Sorts/InsertionSort.java)
- [C](https://github.com/TheAlgorithms/C/blob/master/sorting/insertion_sort.c)
- [C++](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/sorting/insertion_sort.cpp)
- [C#](https://github.com/TheAlgorithms/C-Sharp/blob/master/Algorithms/Sorters/Comparison/InsertionSorter.cs)
- [Scala](https://github.com/TheAlgorithms/Scala/blob/master/src/main/scala/Sort/InsertionSort.scala)
- [Python](https://github.com/TheAlgorithms/Python/blob/master/sorts/insertion_sort.py)
- [Ruby](https://github.com/TheAlgorithms/Ruby/blob/master/sorting/insertion_sort.rb)

#### Video Explanation

[A CS50 video explaining the Insertion Search Algorithm](https://www.youtube.com/watch?v=DFG-XuyPYUQ)


Insertion Sort

Pancake Sort

Patience Sort


#### Problem Statement

Given an unsorted array of n elements, write a function to sort the array
#### Approach

- Make the right-most index value pivot
- partition the array using pivot value
- quicksort left partition recursively
- quicksort right partition recursively

#### Time Complexity

- `O(n^2)` Worst case performance
- `O(n log n)` Best-case performance
- `O(n log n)` Average performance

#### Space Complexity

`O(log n)` Worst case

#### Founder's Name

Tony Hoare in 1959

#### Example

```
arr[] = {10, 80, 30, 90, 40, 50, 70}
Indexes:  0   1   2   3   4   5   6

low = 0, high =  6, pivot = arr[h] = 70
Initialize index of smaller element, i = -1

Traverse elements from j = low to high-1
j = 0 : Since arr[j] <= pivot, do i++ and swap(arr[i], arr[j])
i = 0
arr[] = {10, 80, 30, 90, 40, 50, 70} // No change as i and j
                                     // are same

j = 1 : Since arr[j] > pivot, do nothing
// No change in i and arr[]

j = 2 : Since arr[j] <= pivot, do i++ and swap(arr[i], arr[j])
i = 1
arr[] = {10, 30, 80, 90, 40, 50, 70} // We swap 80 and 30

j = 3 : Since arr[j] > pivot, do nothing
// No change in i and arr[]

j = 4 : Since arr[j] <= pivot, do i++ and swap(arr[i], arr[j])
i = 2
arr[] = {10, 30, 40, 90, 80, 50, 70} // 80 and 40 Swapped
j = 5 : Since arr[j] <= pivot, do i++ and swap arr[i] with arr[j]
i = 3
arr[] = {10, 30, 40, 50, 80, 90, 70} // 90 and 50 Swapped

We come out of loop because j is now equal to high-1.
Finally we place pivot at correct position by swapping
arr[i+1] and arr[high] (or pivot)
arr[] = {10, 30, 40, 50, 70, 90, 80} // 80 and 70 Swapped

Now 70 is at its correct place. All elements smaller than
70 are before it and all elements greater than 70 are after
it.
```

#### Code Implementation Links

- [Java](https://github.com/TheAlgorithms/Java/blob/master/Sorts/QuickSort.java)
- [C++](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/Sorting/Quick%20Sort.cpp)
- [Python](https://github.com/TheAlgorithms/Python/blob/master/sorts/quick_sort.py)
- [Ruby](https://github.com/TheAlgorithms/Ruby/blob/master/sorting/quicksort.rb)

#### Video Explanation

[A video explaining the Quick Sort Algorithm](https://www.youtube.com/watch?v=COk73cpQbFQ)


Quick Sort


The lower bound for Comparison based sorting algorithm (Merge Sort, Heap Sort, Quick-Sort .. etc) is `Î©(nlogn)`, i.e., they cannot do better than `nlogn`. 

Counting sort is a linear time sorting algorithm that sort in `O(n+k)` time when elements are in the range from 1 to k.

What if the elements are in the range from 1 to n2? We canât use counting sort because counting sort will take `O(n2)` which is worse than comparison-based sorting algorithms. Can we sort such an array in linear time? 

Radix Sort is the answer. The idea of Radix Sort is to do digit by digit sort starting from least significant digit to most significant digit. Radix sort uses counting sort as a subroutine to sort.

## The Radix Sort Algorithm 

Do following for each digit i where i varies from least significant digit to the most significant digit. 
Sort input array using counting sort (or any stable sort) according to the iâth digit.

Example:

Original, unsorted list:
`170, 45, 75, 90, 802, 24, 2, 66`

Sorting by least significant digit (1s place) gives:

[*Notice that we keep 802 before 2, because 802 occurred 
before 2 in the original list, and similarly for pairs 
170 & 90 and 45 & 75.]

Sorting by next digit (10s place) gives: 

[*Notice that 802 again comes before 2 as 802 comes before 2 in the previous list.]

`802, 2, 24, 45, 66, 170, 75, 90`

Sorting by the most significant digit (100s place) gives:
`2, 24, 45, 66, 75, 90, 170, 802`

## What is the running time of Radix Sort? 

Let there be d digits in input integers. Radix Sort takes `O(d*(n+b))` time where b is the base for representing numbers, for example, for the decimal system, b is 10.
What is the value of d? If `k` is the maximum possible value, then d would be `O(logb(k))`. So overall time complexity is `O((n+b) * logb(k))`. Which looks more than the 
time complexity of comparison-based sorting algorithms for a large k. Let us first limit k. Let k <= nc where c is a constant. In that case, the complexity becomes 
`O(n logb(n))`. But it still doesnât beat comparison-based sorting algorithms. 

## Is Radix Sort preferable to Comparison based sorting algorithms like Quick-Sort? 

If we have `log2n` bits for every digit, the running time of Radix appears to be better than Quick Sort for a wide range of input numbers. The constant factors hidden in 
asymptotic notation are higher for Radix Sort and Quick-Sort uses hardware caches more effectively. Also, Radix sort uses counting sort as a subroutine and counting sort
takes extra space to sort numbers.

Video reference: https://youtu.be/nu4gDuFabIM


Radix Sort


#### Problem Statement

Given an unsorted array of n elements, write a function to sort the array

#### Approach

- select the smallest element from the array
- put it at the beginning of the array
- then select the smallest array from the remaining unsorted list
- append it to the sorted array at the beginning
- keep doing this for every element of the array
- repeat the above process n times

#### Time Complexity

`O(n^2)` Worst case performance

`O(n^2)` Best-case performance

`O(n^2)` Average performance

#### Space Complexity

`O(1)` Worst case

#### Example

```
arr[] = {80, 10, 40, 30}
Indexes: 0   1   2   3    

1. Index = 0
	Select the minimum number from the array (between index 0-3), ie, 10
2. Swap 10  and 80 (arr[0])
3. The array now is {10, 80, 40, 30}

4. Index = 1
	Select the minimum number from the array (between index 1-3), ie, 30
5. Swap 30 and 80 (arr[1])
6. The array now is {10, 30, 40, 80}

7. Index = 2
	Select the minimum number from the array (between index 2-3), ie, 40
8. Swap 40 and 40 (arr[2])
9. The array now is {10, 30, 40, 80}

The array is now sorted.
```

#### Code Implementation Links

- [Java](https://github.com/TheAlgorithms/Java/blob/master/Sorts/SelectionSort.java)
- [C++](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/Sorting/Selection%20Sort.cpp)
- [Python](https://github.com/TheAlgorithms/Python/blob/master/sorts/selection_sort.py)
- [Go](https://github.com/TheAlgorithms/Go/blob/master/sorts/selection_sort.go)
- [Ruby](https://github.com/TheAlgorithms/Ruby/blob/master/Sorting/selection_sort.rb)
- [C](https://github.com/TheAlgorithms/C/blob/master/sorting/SelectionSort.c)
- [Scala](https://github.com/TheAlgorithms/Scala/blob/master/src/main/scala/Sort/SelectionSort.scala)
- [Javascript](https://github.com/TheAlgorithms/Javascript/blob/master/Sorts/selectionSort.js)

#### Video Explanation

[A video explaining the Selection Sort Algorithm](https://www.youtube.com/watch?v=f8hXR_Hvybo)

#### Animation Explanation

- [Tute Board](https://boardhub.github.io/tute/?wd=selectSortAlgo2)


Selection Sort


#### Problem Statement

Given an unsorted array of n elements, write a function to sort the array

#### Approach

- start with the initial gap, g
- go through the first (n - g) elements in the array  
- compare the element with the next element that is g distance away
- swap the two elements if the first element is bigger
- decrease the gap and repeat until gap = 1

#### Time Complexity
Time complexity is dependent on the gap sequences.
Below time complexities are based on the gap sequences of n/2^k.

`O(n^2)` Worst case performance

`O(n)` Best-case performance

`O(n^2)` Average performance

#### Space Complexity

`O(1)` Worst case

#### Founder's Name

Donald Shell

#### Example

```
arr[] = {61, 109, 149, 111, 34, 2, 24, 119}
Initial Gap: 4   

1.  Index = 0, Next element index = 4
2.  61 > 34, swap 61 and 34
3.  The array is now {34, 109, 149, 111, 61, 2, 24, 119}

4.  Index = 1, Next element index = 5
5.  109 > 2, swap 109 and 2
6.  The array is now {34, 2, 149, 111, 61, 109, 24, 119}

7.  Index = 2, Next element index = 6
8.  149 > 24, swap 149 and 24
9.  The array is now {34, 2, 24, 111, 61, 109, 149, 119}

10. Index = 3, Next element index = 7
11. 111 < 119, do nothing and continue

12. Divide the gap by 2 and repeat until gap = 1
```

#### Code Implementation Links

- [Java](https://github.com/TheAlgorithms/Java/blob/master/Sorts/ShellSort.java)
- [C++](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/Sorting/Shell%20Sort.cpp)
- [Python](https://github.com/TheAlgorithms/Python/blob/master/sorts/shell_sort.py)
- [C-Sharp](https://github.com/TheAlgorithms/C-Sharp/blob/master/sorts/shell_sort.cs)
- [Go](https://github.com/TheAlgorithms/Go/blob/master/sorts/shell_sort.go)
- [Ruby](https://github.com/TheAlgorithms/Ruby/blob/master/Sorting/shell_sort.rb)
- [C](https://github.com/TheAlgorithms/C/blob/master/sorting/shellSort.c)
- [Javascript](https://github.com/TheAlgorithms/Javascript/blob/master/Sorts/shellSort.js)

#### Video Explanation

[A video explaining the Shell Sort Algorithm](https://www.youtube.com/watch?v=H8NiFkGu2PY)

#### Others

Shell sort is also known as diminishing increment sort.


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