Added Merge Sort Exercise Code

Author: RohanSaxena2020

5.py

@@ -0,0 +1,158 @@
+# ### Merge Sort Exercise
+
+# Modify [merge_sort function](https://github.com/codebasics/data-structures-algorithms-python/blob/master/algorithms/5_MergeSort/merge_sort_final.py) such that it can sort following list of athletes as per the time taken by them in the marathon,
+# ```
+# elements = [
+#         { 'name': 'vedanth',   'age': 17, 'time_hours': 1},
+#         { 'name': 'rajab', 'age': 12,  'time_hours': 3},
+#         { 'name': 'vignesh',  'age': 21,  'time_hours': 2.5},
+#         { 'name': 'chinmay',  'age': 24,  'time_hours': 1.5},
+#     ]
+# ``` 
+# merge_sort function should take key from an athlete's marathon log and sort the list as per that key. For example,
+# ```
+# merge_sort(elements, key='time_hours', descending=True)
+# ```
+# This will sort elements by time_hours and your sorted list will look like,
+# ```
+# elements = [
+#         {'name': 'rajab', 'age': 12, 'time_hours': 3},
+#         {'name': 'vignesh', 'age': 21, 'time_hours': 2.5},
+#         {'name': 'chinmay', 'age': 24, 'time_hours': 1.5},
+#         {'name': 'vedanth', 'age': 17, 'time_hours': 1},
+#     ]
+# ``` 
+# But if you call it like this,
+# ```
+# merge_sort(elements, key='name')
+# ```
+# output will be,
+# ```
+# elements = [
+#         { 'name': 'chinmay',   'age': 24, 'time_hours': 1.5},
+#         { 'name': 'rajab', 'age': 12,  'time_hours': 3},
+#         { 'name': 'vedanth',  'age': 17,  'time_hours': 1},
+#         { 'name': 'vignesh',  'age': 21,  'time_hours': 2.5},
+#     ]
+# ``` 
+
+# [Solution](https://github.com/codebasics/data-structures-algorithms-python/blob/master/algorithms/5_MergeSort/merge_sort_exercise_solution.py)
+
+# def merge_sort(arr):
+#     if len(arr) <= 1:
+#         return
+
+#     mid = len(arr)//2
+
+#     left = arr[:mid]
+#     right = arr[mid:]
+
+#     merge_sort(left)
+#     merge_sort(right)
+
+#     merge_two_sorted_lists(left, right, arr)
+
+# def merge_two_sorted_lists(a,b,arr):
+#     len_a = len(a)
+#     len_b = len(b)
+
+#     i = j = k = 0
+
+#     while i < len_a and j < len_b:
+#         if a[i] <= b[j]:
+#             arr[k] = a[i]
+#             i+=1
+#         else:
+#             arr[k] = b[j]
+#             j+=1
+#         k+=1
+
+#     while i < len_a:
+#         arr[k] = a[i]
+#         i+=1
+#         k+=1
+
+#     while j < len_b:
+#         arr[k] = b[j]
+#         j+=1
+#         k+=1
+
+# def merge_two_sorted_by_key(a,b,arr,key):
+#     len_a = len(a)
+#     len_b = len(b)
+
+#     i = j = k = 0
+
+#     while i < len_a and j < len_b:
+#         if a[i][key] <= b[j][key]:
+#             arr[k] = a[i]
+#             i+=1
+#         else:
+#             arr[k] = b[j]
+#             j+=1
+#         k+=1
+
+#     while i < len_a:
+#         arr[k] = a[i]
+#         i+=1
+#         k+=1
+
+#     while j < len_b:
+#         arr[k] = b[j]
+#         j+=1
+#         k+=1
+
+def merge_sort_by_key(arr, key,descending=False):
+    if len(arr) <= 1:
+        return arr
+
+    mid = len(arr)//2
+
+    left = arr[:mid]
+    right = arr[mid:]
+
+    left = merge_sort_by_key(left,key,descending)
+    right = merge_sort_by_key(right,key,descending)
+
+    return merge_two_sorted_lists_by_key(left, right,key,descending)
+
+def merge_two_sorted_lists_by_key(a,b,key,descending):
+    sorted_list = []
+
+    len_a = len(a)
+    len_b = len(b)
+
+    i = j = 0
+
+    while i < len_a and j < len_b:
+        if descending:
+            condition = a[i][key] > b[j][key]  # Note the change here for descending
+        else:
+            condition = a[i][key] <= b[j][key]
+
+        if condition:
+            sorted_list.append(a[i])
+            i += 1
+        else:
+            sorted_list.append(b[j])
+            j += 1
+
+    while i < len_a:
+        sorted_list.append(a[i])
+        i+=1
+
+    while j < len_b:
+        sorted_list.append(b[j])
+        j+=1
+
+    return sorted_list
+
+
+elements = [
+    { 'name': 'vedanth',   'age': 17, 'time_hours': 1},
+    { 'name': 'rajab', 'age': 12,  'time_hours': 3},
+    { 'name': 'vignesh',  'age': 21,  'time_hours': 2.5},
+    { 'name': 'chinmay',  'age': 24,  'time_hours': 1.5},
+   ]
+
+print(merge_sort_by_key(elements,key="age",descending=False))