Valid Anagram: the idea to remember.

The core insight

Count characters in the first string and subtract characters in the second.

Why it works

Every bucket is a frequency difference. All zero means the two multisets match.

The mistake to avoid

A set loses multiplicity: aab and abb have the same set but are not anagrams.

Commented Java solution

Use java.util.* imports and a surrounding solution class. Tree snippets assume the standard TreeNode definition; linked lists use ListNode and Clone Graph uses a Node with val and neighbors; Copy Random List uses Node with val, next and random. Read the input assumptions above.

boolean isAnagram(String s, String t) {
    if (s.length() != t.length()) return false;
    int[] count = new int[26]; // Lowercase English input.
    for (int i = 0; i < s.length(); i++) {
        count[s.charAt(i) - 'a']++;
        count[t.charAt(i) - 'a']--;
    }
    for (int c : count) if (c != 0) return false;
    return true;
}

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