Given a list of integers, find the equilibrium index where the sum of elements to the left equals the sum of elements to the right. The element at the equilibrium index is not included in either sum. Return -1 if no equilibrium index exists. Return -1 for null or empty input.

Examples:

Input: nums = (1, 3, 5, 2, 2) Output: 2 Explanation: Left sum = 1+3 = 4, Right sum = 2+2 = 4, equilibrium at index 2 Input: nums = (0, 1, -1) Output: 0 Explanation: Left sum = 0 (nothing to the left), Right sum = 1+(-1) = 0 Input: nums = (1, 2, 3) Output: -1 Explanation: No index where left sum equals right sum Input: nums = (5) Output: 0 Explanation: Single element always has equilibrium at index 0 Input: nums = null Output: -1 Explanation: Null input returns -1
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