Python | Add list at beginning of list

Sometimes, while working with Python list, we have a problem in which we need to add a complete list to another. The rear end addition to list has been discussed before. But sometimes, we need to perform an append at beginning of list. Let's discuss certain ways in which this task can be performed.

Method #1 : Using "+" operator The "+" operator can be used to perform this particular task. In this, we just perform the addition of one list before other and construct a new list or perform the addition to same list. 

The original list is : [1, 4, 5, 7, 6]
The add list is : [3, 4, 2, 10]
The original updated list is : [3, 4, 2, 10, 1, 4, 5, 7, 6]

Time Complexity: O(n), where n is the length of the list test_list 
Auxiliary Space: O(n) additional space of size n is created where n is the number of elements in the add_list 

  Method #2 : Using deque.extendleft() + reversed() This task can also be performed using combination of above methods. In this, we just convert the list into a dequeue, to allow a front append, and then one by one addition is done by extendleft(), the add list is reversed so that addition take place in correct order using reversed(). 

The original list is : [1, 4, 5, 7, 6]
The add list is : [3, 4, 2, 10]
The original updated list is : [3, 4, 2, 10, 1, 4, 5, 7, 6]

Time Complexity: O(n*n), where n is the length of the list test_list 
Auxiliary Space: O(n) additional space of size n is created where n is the number of elements in the res list 

Method #3 : Using extend() method

The original list is : [1, 4, 5, 7, 6]
The add list is : [3, 4, 2, 10]
The original updated list is : [3, 4, 2, 10, 1, 4, 5, 7, 6]

Time Complexity: O(n*n), where n is the length of the list test_list 
Auxiliary Space: O(n) additional space of size n is created where n is the number of elements in the list 

Method #4 : Using append() method

Approach 

1. Initiate a for loop to traverse test_list
2. Append each element to add_list
3. Display add_list

The original list is : [1, 4, 5, 7, 6]
The add list is : [3, 4, 2, 10]
The original updated list is : [3, 4, 2, 10, 1, 4, 5, 7, 6]

Method #5 : Using insert() method

Approach

• Initiate a for loop to traverse test_list
• Insert each element to add_list using index
• Display add_list

The original list is : [1, 4, 5, 7, 6]
The add list is : [3, 4, 2, 10]
The original updated list is : [1, 4, 5, 7, 6, 3, 4, 2, 10]

Time Complexity: O(N) N length of the list

Auxiliary Space: O(M) M length of add_list