Performance Refactoring

Refactor an existing piece of code to improve its performance.

The function below returns unique values in their original order. Replace the inefficient membership-checking approach while preserving its behavior.

def unique_in_order(items):
    result = []
    for item in items:
        if item not in result:
            result.append(item)
    return result

Input is a list of integers. Return a new list containing the first occurrence of each value.