added sorting alg in python

Author: Zeyu-Li

README.md

@@ -75,6 +75,15 @@ The next two have a node (SLinkedListNode or DLinkedListNode) and the implementa
 
 * Is prime
 
+## Sorting
+
+### Python
+
+* Insertion O(n^2)
+* Bubble O(n^2)
+* Merge O(nlogn)
+* Quick O(nlogn)
+
 ## Concepts
 
 ### Greedy

Regex.md

@@ -29,3 +29,4 @@ Sourced: https://github.com/CoreyMSchafer/code_snippets/blob/master/Regular-Expr
 
 \* Note captured group and be recombined with the **#** followed by the group number
 
+To test out regex use [regex101.com](https://regex101.com/)

python/sort/bubble.py

@@ -0,0 +1,11 @@
+""" Bubble sort O(n^2) """
+
+def bubbleSort(arr):
+    # bubbles the largest number up
+
+    # for all elements in array (skip first element)
+    for i in range(len(arr)):
+        for j in range(len(arr)-i-1):
+            # swap elements if smaller
+            if arr[j] > arr[j+1]:
+                arr[j], arr[j+1] = arr[j+1], arr[j]

python/sort/insertion.py

@@ -0,0 +1,15 @@
+""" Insertion sort O(n^2) """
+
+def insertionSort(arr):
+    # for all elements in array (skip first element)
+    for i in range(1, len(arr)):
+        # examining element
+        key = arr[i]
+
+        # start from the one below the current and move down the array
+        j = i
+        while j > 0 and key < arr[j-1]:
+            # shift one down
+            arr[j] = arr[j-1]
+            j-=1
+        arr[j] = key

python/sort/merge.py

@@ -0,0 +1,32 @@
+""" Merge sort O(nlogn) """
+
+def merge(left, right):
+    i = j = 0
+    result = []
+
+    # merging left and right mantaining sorted order
+    while i < len(left) and j < len(right):
+        if left[i] < right[j]:
+            result.append(left[i])
+            i+=1
+        else:
+            result.append(right[j])
+            j+=1
+
+    # append the rest of the subarray into result
+    result += left[i:]
+    result += right[j:]
+    return result
+
+def mergeSort(arr):
+    if len(arr) <= 1:
+        return arr
+    
+    mid = len(arr)//2
+
+    # have a left and right hand side
+    # sort left and right sublist
+    left = mergeSort(arr[:mid])
+    right = mergeSort(arr[mid:])
+
+    return merge(left, right)

python/sort/quick.py

@@ -0,0 +1,27 @@
+""" Quick sort O(nlogn) """
+
+# returns sorted array (not as space efficient as in array quick sort)
+def quickSort(arr):
+    # return if trivial cases
+    if len(arr) <= 1:
+        return arr
+    
+    # pivot
+    pivot = arr[0]
+    i = 0
+    # for all elements in array
+    for j in range(len(arr) - 1):
+        # swap to end if larger than pivot 
+        if arr[j+1] < pivot:
+            # swap
+            arr[j+1], arr[i+1] = arr[i+1], arr[j+1]
+            i+=1
+
+    # swap pivot
+    arr[0], arr[i] = arr[i], arr[0]
+    left = quickSort(arr[:i])
+    right = quickSort(arr[i+1:])
+    # add pivot back and return the whole subarray
+    left.append(arr[i])
+
+    return left + right