added merge sort algorithm

CONTRIBUTORS.md

@@ -1,5 +1,6 @@
 # Contributors ✨
 
-Thanks to everyone contributing to this repository!  
+Thanks to everyone contributing to this repository!
 
-- [Your Name Here]  
+- [Your Name Here]
+- [Babar Mohiudin Mir]

Merge-Sort.py

@@ -0,0 +1,49 @@
+# this code explains the merge sorting algorithm with help of leetcode problem number 912
+
+def merge(nums, l, mid, r):  # function for merging sub arrays
+    a = []
+    b = []
+    for i in range(l, mid+1):  # itetrate from 0 to mid elements of nums(array) and add into a
+        a.append(nums[i])
+    for i in range(mid+1, r+1):  # itetrate from  mid+1 to end  elements of nums(array) and add into b
+        b.append(nums[i])
+    i, j, k = 0, 0, l  # creating pointers
+
+    while (k <= r):   # merging a and b into nums
+        if j == len(b):
+            nums[k] = a[i]
+            i += 1
+            k += 1
+        elif i == len(a):
+            nums[k] == b[j]
+            j += 1
+            k += 1
+        elif a[i] < b[j]:
+            nums[k] = a[i]
+            i += 1
+            k += 1
+        else:
+            nums[k] = b[j]
+            j += 1
+            k += 1
+
+
+def mergesort(nums, l, r):  # functions that break problem into subproblems(recursion)
+    # base case
+    if l >= r:
+        return
+    # recursive case
+    mid = (l+r)//2
+    mergesort(nums, l, mid)
+    mergesort(nums, mid+1, r)
+    merge(nums, l, mid, r)
+
+
+def sortary(nums):  # main function
+    n = len(nums)-1
+    mergesort(nums, 0, n)
+    return nums
+
+
+# calling a main function
+print(sortary([3, 53, 25, 4, 25, 66, 4, 4, 3, 5, 6, 42, 55, 3245,]))