해커랭크 https://www.hackerrank.com
프로그래머스 https://programmers.co.kr

코드업 https://codeup.kr
백준 https://www.acmicpc.net
코드잇 https://www.codeit.kr/
렛코드 https://leetcode.com/

SW Expert Academy
Programming Online Training In Coding Skills | Codility
Advent of Code 2019
https://www.codeground.org
https://www.swexpertacademy.com
https://algospot.com/judge/problem/list/

사이냅: http://euler.synap.co.kr/
Topcoder: https://www.topcoder.com/
알고스팟: https://algospot.com/judge/problem/list/
geeksforgeeks: https://www.geeksforgeeks.org/
CodeForces: http://codeforces.com
더블릿: http://59.23.150.58/
HackerEarth: https://www.hackerearth.com/

위 링크중 가본 곳.
해커링크, 프로그래머스

출처

https://gmlwjd9405.github.io/2018/05/14/how-to-study-algorithms.html