Basic partition
Input
nums = [3, 1, 2, 4, 7, 6]
Output
[6, 4, 2, 1, 7, 3]
A two-pointer partition swaps odd numbers on the left with even numbers on the right. Evens (6,4,2) appear before odds (1,7,3).
Full lesson preview
Move even numbers before odd numbers in-place
Partition an integer list in-place so that all even numbers appear before odd numbers using a two-pointer approach.
Python practice15 minTwo Pointers & Sliding WindowIntermediateLast updated March 26, 2026
Problem statement
Given a list of integers nums, reorder the elements in-place so that all even numbers come before all odd numbers. The relative order among even numbers or among odd numbers does not need to be preserved. Return the same list object after partitioning.
You must perform the operation in-place using O(1) additional memory and in linear time.
Task
Use two pointers to rearrange an array in-place so that every even number is positioned before any odd number. Achieve O(n) time and O(1) extra space.
Examples
Input format
A single function call move_even_before_odd(nums) where nums is a list of integers.
Output format
Return the same list object after reordering so that all even integers appear before all odd integers.
Constraints
0 <= len(nums) <= 10^5; integers can be negative, zero, or positive. In-place (O(1) extra space) and O(n) time required.
Samples
Sample 1
Input
[2, 4, 6]
Output
[2, 4, 6]
All elements are even; the list remains the same.