Daily Challenges

One focused problem per day. Small enough to do before work, hard enough to be worth doing.

How it works

Each file has the problem at the top and the answer collapsed underneath, in a <details markdown="1"> block. GitHub renders that as a "Show solution" toggle, so you can read the problem without seeing the answer.

Do this:

Read the problem and the constraints.
Set a 20-minute timer and write the code in your own editor.
Stuck at 10 minutes? Expand Hint only.
Finished, or out of time? Expand Solution and compare — especially the complexity and the edge cases.

Time-boxing matters more than finishing. An hour of grinding teaches less than 20 minutes followed by reading a good solution.

Layout

daily-challenges/
├── TEMPLATE.md          copy this to add a new day
└── 2026/
    └── 09/
        └── 2026-09-17.md

One file per day, named YYYY-MM-DD.md, filed under YYYY/MM/.

Index

Date	Challenge	Difficulty	Pattern
2026-09-17	Move zeroes to the end, in place	🟢 Easy	Two pointers
2026-09-18	Product of array except self	🟡 Medium	Prefix/suffix products
2026-09-19	Find the duplicate number	🔴 Hard	Floyd's cycle detection
2026-09-20	First unique character in a string	🟢 Easy	Hash map (count, then scan)
2026-09-21	Next permutation	🟡 Medium	In-place array manipulation
2026-09-22	Trapping rain water	🔴 Hard	Two pointers + running maxima
2026-09-23	Interleaving string	🟡 Medium	DP over two inputs
2026-09-24	Longest valid parentheses	🔴 Hard	Stack of indices with a base sentinel
2026-09-25	Valid Sudoku	🟢 Easy	Constrained counting (row, column, box)

Contributing a day

Copy TEMPLATE.md, fill it in, and add a row to the index above. The solution must be correct and its complexity stated honestly — see CONTRIBUTING.md.

Found a mistake? Edit this page on GitHub.