Learn DSA one stage at a time

Why this stage

Arrays and strings are the default input shape on LeetCode. If you can scan a list, count characters, and use a hash map for lookups, the later patterns feel much easier.

What you learn

• Index into a list and walk left → right
• Count frequencies with a hash map
• Use a set to check “have I seen this?”
• Build a new string or array from an answer

Starter problems

• Two SumLeetCode+9Easy
• Contains DuplicateNeetCode+9Easy
• Valid AnagramNeetCode+9Easy

Tips

• →Say the brute force out loud first, then ask “what am I looking up twice?”
• →Prefer a hash map when you need O(1) lookups by value.