--- title: Stop Memorizing the Dutch National Flag Algorithm -... description: A first-principles walkthrough of the Dutch National Flag algorithm (LeetCode 75: Sort Colors). Rather than memorizing pointer movements, the author frames the... canonical: https://daily.dev/posts/stop-memorizing-the-dutch-national-flag-algorithm---learn-to-derive-it-instead-leetcode-75--lbqc8fr6v twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Stop Memorizing the Dutch National Flag Algorithm - Learn to Derive It Instead (LeetCode 75) | daily.dev og:description: A first-principles walkthrough of the Dutch National Flag algorithm (LeetCode 75: Sort Colors). Rather than memorizing pointer movements, the author frames the... og:url: https://daily.dev/posts/stop-memorizing-the-dutch-national-flag-algorithm---learn-to-derive-it-instead-leetcode-75--lbqc8fr6v og:image: https://api.daily.dev/og/posts/LBQc8fr6V.png og:image:alt: Stop Memorizing the Dutch National Flag Algorithm - Learn to Derive It Instead (LeetCode 75) og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Stop Memorizing the Dutch National Flag Algorithm - Learn to Derive It Instead (LeetCode 75) **[Medium](https://daily.dev/sources/medium_js)** · 6 min read · 0 upvotes · 0 comments ## Summary A first-principles walkthrough of the Dutch National Flag algorithm (LeetCode 75: Sort Colors). Rather than memorizing pointer movements, the author frames the problem as maintaining four distinct regions in the array — confirmed 0s, confirmed 1s, unknown, and confirmed 2s — and shrinking the unknown region one element at a time. The post explains why mid doesn't advance after swapping with right, why the loop condition is mid <= right rather than left <= right, and lists common mistakes. Includes a complete Python implementation with O(n) time and O(1) space complexity. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Why doesn't the mid pointer move after swapping with the right pointer in the Dutch National Flag algorithm? The mid pointer stays in place because the value swapped in from the right position has not yet been classified. Since it could be a 0, 1, or 2, it must be inspected again on the next iteration; moving mid forward would skip classifying that element and break the algorithm's correctness. _Developers untangling three-pointer sorting logic often revisit worked explanations like this one via daily.dev._ ### What should the loop condition be for the Dutch National Flag three-pointer sorting algorithm? The loop should run while mid <= right, not while left <= right. The unknown region being processed lies between mid and right, so the loop must continue until that region is empty; stopping based on left and right alone can leave the last unknown element unsorted, as shown with the input [1,0]. _Interview prep on pointer-based algorithms is easier to revisit later through daily.dev._ ### What are the time and space complexity of the Dutch National Flag sort colors algorithm? The algorithm runs in O(n) time and O(1) space. Each element in the unknown region is classified and moved a constant number of times, and the array is sorted in-place using three pointers (left, mid, right) without extra memory allocation. _Track algorithm complexity trade-offs for interview prep alongside daily.dev._ ## Similar posts on daily.dev - [Day 11: Two Pointers — Sort Colors \(Dutch National Flag\)](https://daily.dev/posts/day-11-two-pointers-sort-colors-dutch-national-flag--l4r1ilvs0) · Medium · 2 upvotes · 0 comments - [Color Wheel combinatorics](https://daily.dev/posts/color-wheel-combinatorics-jpovtpbzj) · Arthur O'Dwyer · 0 upvotes · 0 comments - [January 2, 2026](https://daily.dev/posts/january-2-2026-60wwwmhvi) · DevBlogs · 0 upvotes · 0 comments --- Tags: [#python](https://daily.dev/tags/python), [#algorithms](https://daily.dev/tags/algorithms), [#interview-questions](https://daily.dev/tags/interview-questions) [View this post on daily.dev](https://daily.dev/posts/stop-memorizing-the-dutch-national-flag-algorithm---learn-to-derive-it-instead-leetcode-75--lbqc8fr6v) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"Stop Memorizing the Dutch National Flag Algorithm - Learn to Derive It Instead (LeetCode 75)","url":"https://daily.dev/posts/stop-memorizing-the-dutch-national-flag-algorithm---learn-to-derive-it-instead-leetcode-75--lbqc8fr6v","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/stop-memorizing-the-dutch-national-flag-algorithm---learn-to-derive-it-instead-leetcode-75--lbqc8fr6v"},"datePublished":"2026-07-22T06:10:49.196Z","dateModified":"2026-09-14T08:57:15.601Z","description":"A first-principles walkthrough of the Dutch National Flag algorithm (LeetCode 75: Sort Colors). 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