Find the Equilibrium Index of an Array. Reference: https:www.geeksforgeeks.orgequilibriumindexofanarray Python doctest can be run with the following command: python m doctest v equilibriumindex.py Given a sequence arr of size n, this function returns an equilibrium index if any or 1 if no equilibrium index exists. The equilibrium index of an array is an index such that the sum of elements at lower indexes is equal to the sum of elements at higher indexes. Example Input: arr  7, 1, 5, 2, 4, 3, 0 Output: 3 Find the equilibrium index of an array. Args: arr listint: The input array of integers. Returns: int: The equilibrium index or 1 if no equilibrium index exists. Examples:  equilibriumindex7, 1, 5, 2, 4, 3, 0 3  equilibriumindex1, 2, 3, 4, 5 1  equilibriumindex1, 1, 1, 1, 1 2  equilibriumindex2, 4, 6, 8, 10, 3 1
def equilibrium_index(arr: list[int]) -> int:
    """
    Find the equilibrium index of an array.

    Args:
        arr (list[int]): The input array of integers.

    Returns:
        int: The equilibrium index or -1 if no equilibrium index exists.

    Examples:
        >>> equilibrium_index([-7, 1, 5, 2, -4, 3, 0])
        3
        >>> equilibrium_index([1, 2, 3, 4, 5])
        -1
        >>> equilibrium_index([1, 1, 1, 1, 1])
        2
        >>> equilibrium_index([2, 4, 6, 8, 10, 3])
        -1
    """
    total_sum = sum(arr)
    left_sum = 0

    for i, value in enumerate(arr):
        total_sum -= value
        if left_sum == total_sum:
            return i
        left_sum += value

    return -1


if __name__ == "__main__":
    import doctest

    doctest.testmod()
