Even lengths
Input
a = [1, 3], b = [2, 4]
Output
[1, 2, 3, 4]
Elements alternate: 1,2,3,4.
Full lesson preview
Interleave two lists element by element
Combine two lists by alternating elements from each. If lists differ in length, append remaining elements.
Python practice15 minLists & TuplesIntermediateLast updated March 17, 2026
Problem statement
Write a function interleave_lists(a, b) that takes two lists a and b and returns a new list formed by taking elements alternately from a and b: first element of a, first of b, second of a, second of b, and so on. If one list is longer, append the remaining tail of the longer list at the end in order.
Both inputs are lists and may contain any types. Do not modify the input lists.
Task
Implement interleaving of two lists into a single list with elements taken alternately.
Examples
Input format
Two Python lists a and b.
Output format
Return a single list with elements interleaved and remaining elements appended if lengths differ.
Constraints
Do not use external libraries. Must preserve element order. Time complexity O(n) where n = len(a)+len(b).
Samples
Sample 1
Input
a = [1, 2, 3], b = ['x']
Output
[1, 'x', 2, 3]
After taking 1 and 'x', append remaining 2 and 3.