Determine whether one string is an anagram of another.

Interview question: Valid Anagram

Given two strings `s` and `t`, return `true` if `t` is an anagram of `s`, and `false` otherwise.

An anagram is a word formed by rearranging the letters of another, using all the original letters exactly once.

- Two strings are anagrams only when they use the same characters with the same frequencies.
- Rearranging is allowed, but adding, removing, or changing a character is not.
- If the lengths differ, they cannot be anagrams.

Understand the Requirement

- Sorting both strings puts equal characters next to each other.
- If the sorted strings are equal, the original strings contain the same letters.
- This works, but sorting costs O(n log n) time.

Start with a Sorting Approach

- A faster approach is to count how many times each character appears.
- Increment counts for characters in `s`.
- Decrement counts for characters in `t`; every count should return to zero.

Use Character Counts Instead

- Length checking avoids unnecessary work.
- If one string has extra characters, no count-based comparison can succeed.
- After this guard, each index in `s` can be paired with the same index in `t` for counting.

Reject Different Lengths Early

- The dictionary stores the frequency balance for each character.
- Any nonzero final count means one string used that character more often.
- Time complexity is O(n), and space complexity is O(k), where `k` is the number of distinct characters.