Remove Duplicates Preserving Order

Problem Given an array, return its elements with duplicates removed, keeping each value's first-occurrence order. Sorting is not allowed, since it would destroy the original order.

Input / Output

• Input: array arr of n elements.
• Output: the distinct elements in the order they first appear.

Constraints

• 1 <= n <= 10^5.
• Preserve original relative order; do not sort.

Example

• [3,1,3,4,1,5] -> [3,1,4,5]
• [7,7,7] -> [7]