Merge pull request codebasics#4 from codebasics/merge-sort-exercise

Merge Sort Exercises

Author: groverkds

algorithms/5_MergeSort/merge_sort_exercise.md

@@ -0,0 +1,40 @@
+### Bubble Sort Exercise
+
+Modify [merge_sort function](https://github.com/codebasics/py/blob/master/Algorithms/5_MergeSort/merge_sort.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/py/blob/master/Algorithms/2_BubbleSort/merge_sort_exercise_solution.py)
+

algorithms/5_MergeSort/merge_sort_exercise_solution.py

@@ -0,0 +1,44 @@
+# you can use this to sort strings too
+def merge_sort(elements, key, descending=False):
+    size = len(elements)
+
+    if size == 1:
+        return elements
+
+    left_list = merge_sort(elements[0:size//2], key, descending)
+    right_list = merge_sort(elements[size//2:], key, descending)
+    sorted_list = merge(left_list, right_list, key, descending)
+
+    return sorted_list
+
+    
+def merge(left_list, right_list, key, descending=False):
+    merged = []
+    if descending:
+        while len(left_list) > 0 and len(right_list) > 0:
+            if left_list[0][key] >= right_list[0][key]:
+                merged.append(left_list.pop(0))
+            else:
+                merged.append(right_list.pop(0))
+
+    else:
+        while len(left_list) > 0 and len(right_list) > 0:
+            if left_list[0][key] <= right_list[0][key]:
+                merged.append(left_list.pop(0))
+            else:
+                merged.append(right_list.pop(0))
+
+    merged.extend(left_list)
+    merged.extend(right_list)
+    return merged
+
+if __name__ == '__main__':
+    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},
+    ]
+
+    sorted_list = merge_sort(elements, key='time_hours', descending=True)
+    print(sorted_list)