Contains Duplicate: the idea to remember.

The core insight

Insert values into a set; a failed insertion reveals a duplicate.

Why it works

After each iteration the set contains exactly the distinct values from the visited prefix.

The mistake to avoid

Comparing only adjacent values works only after sorting.

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 containsDuplicate(int[] nums) {
    Set<Integer> seen = new HashSet<>();
    for (int n : nums) {
        // add returns false if this value was already present.
        if (!seen.add(n)) return true;
    }
    return false;
}

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