1. Codeforces

The main platform for contests, problem-solving, and community discussions.

Features: Contests, problemset, blogs, virtual participation.

Best for: Practicing timed contests, learning problem patterns, and improving rating.

1. AtCoder

Japanese platform with clear, beginner-friendly problems.

Features: ABC (Beginner), ARC (Regular), AGC (Grand) contests.

Best for: Beginners and stepwise improvement.

1. CSES Problem Set

Organized problem sets by topic (sorting, searching, dynamic programming, graphs).

Best for: Structured learning and step-by-step skill building.

1. LeetCode

Huge database of problems, often tagged by difficulty and topic.

Best for: Learning basic algorithms and data structures, also preparing for interviews.

1. CodeChef

Indian platform with Long Challenges (10-day), Cook-Offs, and Lunchtimes.

Best for: Practicing both long and short contests, beginner-friendly problems.

1. HackerRank

Tutorials and problems categorized by data structures, algorithms, and mathematics.

Best for: Absolute beginners who want guided practice with learning explanations.

1. TopCoder

One of the oldest competitive programming platforms.

Features: SRM contests, marathon matches, tutorials.

Best for: Advanced algorithmic challenges after basics.

1. SPOJ

Huge archive of classical algorithm problems.

Best for: Practicing individual problems at your own pace.

1. USACO Training Gateway

Focuses on algorithmic problem-solving for beginners and intermediate coders.

Best for: Learning recursion, DP, and graph problems in a structured way.

1. HackerEarth

Tutorials and practice problems categorized by difficulty and topic.

Best for: Beginners to intermediate coders and timed contests.