Python | Combining two sorted lists

Difficulty Level : Basic
Last Updated : 24 Dec, 2018

Many a times we encounter a problem where we wish to use the merge function of merge sort and is a classical problem that occurs many times while doing competitive programming. This type of problem when known shorter and compact way to perform them are always quite handy.

Let’s discuss certain ways of combining two sorted list in Python.

Method #1 : Naive Method

Merge operation of merge sort can be performed using the naive method which has also been discussed earlier. We check for the smaller of two element on the current index and increment the index of the list whose no. is encountered. When either of the list gets exhausted, the other list is appended to the end of merged list.

# Python3 code to demonstrate 
# to combine two sorted list 
# using naive method 
  
# initializing lists
test_list1 = [1, 5, 6, 9, 11]
test_list2 = [3, 4, 7, 8, 10]
  
# printing original lists 
print ("The original list 1 is : " + str(test_list1))
print ("The original list 2 is : " + str(test_list2))
  
# using naive method 
# to combine two sorted lists
size_1 = len(test_list1)
size_2 = len(test_list2)
  
res = []
i, j = 0, 0
  
while i < size_1 and j < size_2:
    if test_list1[i] < test_list2[j]:
      res.append(test_list1[i])
      i += 1
  
    else:
      res.append(test_list2[j])
      j += 1
  
res = res + test_list1[i:] + test_list2[j:]
  
# printing result
print ("The combined sorted list is : " + str(res))

Output:

The original list 1 is : [1, 5, 6, 9, 11]
The original list 2 is : [3, 4, 7, 8, 10]
The combined sorted list is : [1, 3, 4, 5, 6, 7, 8, 9, 10, 11]

Method #2 : Using sorted()

This function can be used to perform this task in just a 1 line but will take more time internally. It may have more time complexity as we append one list to another and again sort the resultant list. Should be used if we need to save the coding time.

# Python3 code to demonstrate 
# to combine two sorted list 
# using sorted() 
  
# initializing lists
test_list1 = [1, 5, 6, 9, 11]
test_list2 = [3, 4, 7, 8, 10]
  
# printing original lists 
print ("The original list 1 is : " + str(test_list1))
print ("The original list 2 is : " + str(test_list2))
  
# using sorted()
# to combine two sorted lists
res = sorted(test_list1 + test_list2)
  
# printing result
print ("The combined sorted list is : " + str(res))

Output:

The original list 1 is : [1, 5, 6, 9, 11]
The original list 2 is : [3, 4, 7, 8, 10]
The combined sorted list is : [1, 3, 4, 5, 6, 7, 8, 9, 10, 11]

Method #3 : Using heapq.merge()

Python also offers the inbuilt function to perform this particular task and performs the similar working in background as merge in naive method and should be used when wanting to deal with this kind of problem.

# Python3 code to demonstrate 
# to combine two sorted list 
# using heapq.merge()
from heapq import merge
  
# initializing lists
test_list1 = [1, 5, 6, 9, 11]
test_list2 = [3, 4, 7, 8, 10]
  
# printing original lists 
print ("The original list 1 is : " + str(test_list1))
print ("The original list 2 is : " + str(test_list2))
  
# using heapq.merge()
# to combine two sorted lists
res = list(merge(test_list1, test_list2))
  
# printing result
print ("The combined sorted list is : " + str(res))

Output:

The original list 1 is : [1, 5, 6, 9, 11]
The original list 2 is : [3, 4, 7, 8, 10]
The combined sorted list is : [1, 3, 4, 5, 6, 7, 8, 9, 10, 11]

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