Array+equilibrium.swift

extension Array where Element == Int {
    var equilibriumIndexes:  [Element]? {
        var idxs = [Element]()
        var sum = reduce(0,+)
        var leftSum = 0
        let count = self.count
        for i in 0..<count {
            sum = sum - self[i]
            if leftSum == sum {
                idxs.append(i)
            }
            leftSum = leftSum + self[i]
        }
        return idxs.isEmpty ? nil : idxs
    }
}
[-3,2,-2,1,-2].equilibriumIndexes