Typical example
Input
nums = [2, 0, 2, 1, 1, 0]
Output
[0, 0, 1, 1, 2, 2]
A single-pass in-place algorithm groups 0s, then 1s, then 2s.
Full lesson preview
Sort colors in one pass
Sort an array containing 0s, 1s, and 2s in a single pass using constant extra space.
Python practice22 minSearching & SortingAdvancedLast updated March 25, 2026
Problem statement
Given an array nums with n objects colored red, white, or blue (represented by 0, 1, and 2), sort them in-place so that objects of the same color are adjacent, with the colors in the order 0, 1, and 2. You must solve this in one pass with constant extra space. Return the array after sorting.
Task
Use the Dutch National Flag algorithm to sort an array of 0s, 1s, and 2s in one pass (O(n) time, O(1) space).
Examples
Input format
A Python list nums consisting only of integers 0, 1, and 2.
Output format
The input list nums after in-place sorting (returned as a list).
Constraints
1 <= n <= 10^5 typically. Only values 0, 1, and 2 appear. Use O(n) time and O(1) extra space.
Samples
Sample 1
Input
nums = [2, 0, 1]
Output
[0, 1, 2]
Sorted in-place to [0,1,2].