Longest Consecutive Sequence: the idea to remember.

The core insight

Use a set and extend a run only when its predecessor is absent.

Why it works

Every run has exactly one smallest member. Starting only there means each distinct value is extended over at most once.

The mistake to avoid

Iterate over the set, not the original array, to avoid repeatedly expanding a duplicated start. Guard integer boundaries.

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.

int longestConsecutive(int[] nums) {
    Set<Integer> set = new HashSet<>();
    for (int n : nums) set.add(n);
    int best = 0;
    for (int n : set) {
        if (n != Integer.MIN_VALUE && set.contains(n - 1)) continue;
        int last = n, length = 1;
        while (last != Integer.MAX_VALUE && set.contains(last + 1)) { last++; length++; }
        best = Math.max(best, length);
    }
    return best;
}

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