Added Complexity of Each Algorithm in Sorting/ directory. (trekhleb#76)

* Added Complexity to Bubble Sort README.md

* Added Complexity to Counting-Sort README.md

* Italicized n in Bubble Sort README.md

* Added Complexity to Heap Sort README.md

* Added Complexity to Insertion Sort README.md

* Added Complexity to Merge Sort README.md

* Added Complexity to Quick Sort README.md

* Added Complexity to Radix Sort README.md

* Added Complexity to Selection Sort README.md

* Added Complexity to SHell Sort README.md

Author: mj-z-ali

src/algorithms/sorting/bubble-sort/README.md

@@ -9,6 +9,13 @@ are needed, which indicates that the list is sorted.
 
 ![Algorithm Visualization](https://upload.wikimedia.org/wikipedia/commons/c/c8/Bubble-sort-example-300px.gif)
 
+## Complexity
+
+###### time: worst _O_(_n_<sup>2</sup>), best _O_(_n_), average _O_(_n_<sup>2</sup>)
+
+###### space: worst _O_(1) auxiliary
+
+
 ## References
 
 - [Wikipedia](https://en.wikipedia.org/wiki/Bubble_sort)

src/algorithms/sorting/counting-sort/README.md

@@ -54,6 +54,12 @@ zero.
 
 ![Counting Sort](https://1.bp.blogspot.com/-xPqylngqASY/WLGq3p9n9vI/AAAAAAAAAHM/JHdtXAkJY8wYzDMBXxqarjmhpPhM0u8MACLcB/s1600/ResultArrayCS.gif)
 
+## Complexity
+
+###### time: worst _O_(_n_ + _k_), best _O_(_n_), average _O_(_n_ + _k_) where _n_ is the number of elements in the input array and _k_ is the range of the output.
+
+###### space: worst _O_(_n_ + _k_) 
+
 ## References
 
 - [Wikipedia](https://en.wikipedia.org/wiki/Counting_sort)

src/algorithms/sorting/heap-sort/README.md

@@ -13,6 +13,12 @@ rather than a linear-time search to find the maximum.
 
 ![Algorithm Visualization](https://upload.wikimedia.org/wikipedia/commons/4/4d/Heapsort-example.gif)
 
+## Complexity
+
+###### time: worst _O_(_n log n_), best _O_(_n log n_), average _O_(_n log n_)
+
+###### space: worst _O_(1) auxiliary
+
 ## References
 
 [Wikipedia](https://en.wikipedia.org/wiki/Heapsort)

src/algorithms/sorting/insertion-sort/README.md

@@ -10,6 +10,13 @@ sort.
 
 ![Algorithm Visualization](https://upload.wikimedia.org/wikipedia/commons/0/0f/Insertion-sort-example-300px.gif)
 
+## Complexity
+
+###### time: worst _O_(_n_<sup>2</sup>), best _O_(_n_), average _O_(_n_<sup>2</sup>)
+
+###### space: worst _O_(1) auxiliary
+
+
 ## References
 
 [Wikipedia](https://en.wikipedia.org/wiki/Insertion_sort)

src/algorithms/sorting/merge-sort/README.md

@@ -22,6 +22,12 @@ emulate merge sort (top-down).
 
 ![Merge Sort](https://upload.wikimedia.org/wikipedia/commons/e/e6/Merge_sort_algorithm_diagram.svg)
 
+## Complexity
+
+###### time: average _O_(_n log n_)
+
+###### space: worst _O_(_n_) 
+
 ## References
 
 - [Wikipedia](https://en.wikipedia.org/wiki/Merge_sort)

src/algorithms/sorting/quick-sort/README.md

@@ -23,6 +23,12 @@ The horizontal lines are pivot values.
 
 ![Quicksort](https://upload.wikimedia.org/wikipedia/commons/6/6a/Sorting_quicksort_anim.gif)
 
+## Complexity
+
+###### time: worst _O_(_n_<sup>2</sup>), best _O_(_n log n_), average _O_(_n log n_)
+
+###### space: worst _O_(_n_) auxiliary
+
 ## References
 
 - [Wikipedia](https://en.wikipedia.org/wiki/Quicksort)

src/algorithms/sorting/radix-sort/README.md

@@ -25,6 +25,12 @@ comparison-based sorts (and worse if keys are much longer than `log n`).
 
 ![Radix Sort](https://www.researchgate.net/publication/291086231/figure/fig1/AS:614214452404240@1523451545568/Simplistic-illustration-of-the-steps-performed-in-a-radix-sort-In-this-example-the.png)
 
+## Complexity
+
+###### time: worst _O_(_n_), best _O_(_n_), average _O_(_n_)
+
+###### space: always _O_(_n_)
+
 ## References
 
 - [Wikipedia](https://en.wikipedia.org/wiki/Radix_sort)

src/algorithms/sorting/selection-sort/README.md

@@ -13,6 +13,12 @@ memory is limited.
 
 ![Algorithm Visualization](https://upload.wikimedia.org/wikipedia/commons/9/94/Selection-Sort-Animation.gif)
 
+## Complexity
+
+###### time: worst _O_(_n_<sup>2</sup>), best _O_(_n_<sup>2</sup>), average _O_(_n_<sup>2</sup>)
+
+###### space: _O_(1) auxiliary
+
 ## References
 
 [Wikipedia](https://en.wikipedia.org/wiki/Selection_sort)

src/algorithms/sorting/shell-sort/README.md

@@ -42,6 +42,12 @@ Shell sort uses insertion sort to sort the array.
 
 ![Shellsort](https://www.tutorialspoint.com/data_structures_algorithms/images/shell_sort.jpg)
 
+## Complexity
+
+###### time: best _O_(_n log n_), average - depends on 'gap sequence'.
+
+###### space: worst _O_(_n_) total, _O_(1) auxiliary
+
 ## References
 
 * [Tutorials Point](https://www.tutorialspoint.com/data_structures_algorithms/shell_sort_algorithm.htm)