27.py

# Merges two subarrays of arr[].
# First subarray is arr[l..m]
# Second subarray is arr[m+1..r]
def merge(arr, l, m, r):
    n1 = int(m - l + 1)
    n2 = int(r - m)
 
    # create temp arrays
    L = [0] * (n1)
    R = [0] * (n2)
 
    # Copy data to temp arrays L[] and R[]
    for i in range(0 , n1):
        L[i] = arr[l + i]
 
    for j in range(0 , n2):
        R[j] = arr[m + 1 + j]
 
    # Merge the temp arrays back into arr[l..r]
    i = 0     # Initial index of first subarray
    j = 0     # Initial index of second subarray
    k = l     # Initial index of merged subarray
 
    while i < n1 and j < n2 :
        if L[i] <= R[j]:
            arr[k] = L[i]
            i += 1
        else:
            arr[k] = R[j]
            j += 1
        k += 1
 
    # Copy the remaining elements of L[], if there
    # are any
    while i < n1:
        arr[k] = L[i]
        i += 1
        k += 1
 
    # Copy the remaining elements of R[], if there
    # are any
    while j < n2:
        arr[k] = R[j]
        j += 1
        k += 1
 
# l is for left index and r is right index of the
# sub-array of arr to be sorted
def mergeSort(arr,l,r):
    if l < r:

        # find mid
        m = int((l+(r-1))/2)
 
        # Sort first and second halves
        mergeSort(arr, l, m)
        mergeSort(arr, m+1, r)
        merge(arr, l, m, r)
 
arr = [12, 11, 13, 5, 6, 7]
n = len(arr)
print ("Given array is", arr)
 
mergeSort(arr, 0, n-1)
print ("\n\nSorted array is", arr)