Iterative Merge Sort

Author: shashankchakrawarty98

Arrays-Sorting/src/Iterative_Merge_Sort/Iterative_Merge_Sort.py

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+
+#made by SHASHANK CHAKRAWARTY
+#Time complexity of above iterative function is  Θ(nLogn)
+# Iterative Merge sort (Bottom Up)
+ 
+# Iterative mergesort function to
+# sort arr[0...n-1] 
+def mergeSort(a):
+     
+    current_size = 1
+     
+    # Outer loop for traversing Each 
+    # sub array of current_size
+    while current_size < len(a) - 1:
+         
+        left = 0
+        # Inner loop for merge call 
+        # in a sub array
+        # Each complete Iteration sorts
+        # the iterating sub array
+        while left < len(a)-1:
+             
+            # mid index = left index of 
+            # sub array + current sub 
+            # array size - 1
+            mid = left + current_size - 1
+             
+            # (False result,True result)
+            # [Condition] Can use current_size
+            # if 2 * current_size < len(a)-1
+            # else len(a)-1
+            right = ((2 * current_size + left - 1,
+                    len(a) - 1)[2 * current_size 
+                          + left - 1 > len(a)-1])
+                           
+            # Merge call for each sub array
+            merge(a, left, mid, right)
+            left = left + current_size*2
+             
+        # Increasing sub array size by
+        # multiple of 2
+        current_size = 2 * current_size
+ 
+# Merge Function
+def merge(a, l, m, r):
+#you create 2 sub arrays with length n1 and n2
+    n1 = m - l + 1
+    n2 = r - m
+#2 arrays are created
+    L = [0] * n1
+    R = [0] * n2
+#copy the elements into the left and right array for sorting
+    for i in range(0, n1):
+        L[i] = a[l + i]
+    for i in range(0, n2):
+        R[i] = a[m + i + 1]
+#here it sorts
+    i, j, k = 0, 0, l
+    while i < n1 and j < n2:
+        if L[i] > R[j]:
+            a[k] = R[j]
+            j += 1
+        else:
+            a[k] = L[i]
+            i += 1
+        k += 1
+#and you copy the elements into the arrays i.e Left and Right by comparing it with the size of the array which was declared previously 
+    while i < n1:
+        a[k] = L[i]
+        i += 1
+        k += 1
+ 
+    while j < n2:
+        a[k] = R[j]
+        j += 1
+        k += 1
+ 
+ 
+# Driver code you can change it accordingly
+a = [12, 11, 13, 5, 6, 7]
+print("Before Sorting ")
+print(a)
+ 
+mergeSort(a)
+ 
+print("After Sorting ")
+print(a)