Python – Insertion at the beginning in OrderedDict

Given an ordered dict, write a program to insert items in the beginning of the ordered dict.

Examples: 

Input: 
original_dict = {'a':1, 'b':2}
item to be inserted ('c', 3)

Output:  {'c':3, 'a':1, 'b':2}

Input: 
original_dict = {'akshat':1, 'manjeet':2}
item to be inserted ('nikhil', 3)

Output:  {'nikhil':3, 'akshat':1, 'manjeet':2}

Below are various methods to insert items in starting of ordered dict.

Method #1: Using OrderedDict.move_to_end()  

Resultant Dictionary : OrderedDict([('manjeet', '3'), ('akshat', '1'), ('nikhil', '2')])

Time complexity: O(n), where n is the number of values in the dictionary.
Auxiliary Space: O(1), constant extra space is required

Method #2: Using Naive Approach

This method only works in case of unique keys 

Resultant Dictionary :OrderedDict([('manjeet', '4'), ('akash', '4'), ('akshat', '1'), ('nikhil', '2')])

Method #3: Using OrderedDict.popitem() 

To add a new key-value pair at the beginning of an OrderedDict, we can use popitem() method with a new OrderedDict. This method returns and removes the last key-value pair in the OrderedDict. We can keep popping the last key-value pair and adding it to a new OrderedDict until we get the desired order of insertion.

Resultant Dictionary :OrderedDict([('manjeet', '3'), ('akshat', '1'), ('nikhil', '2')])

Time Complexity: O(n), where N is the number of key-value pairs in the ini_dict
Auxiliary Space: O(n), because we are iterating through all the key-value pairs in ini_dict once.

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