customSubsetMinus.swift

func customSubsetMinus(A A: [Int], B: [Int]) -> [Int] {
    var subset = [Int]()
    
    for a in A {
        
        let occurrences = B.reduce(0, combine: { 
            (sum, b) -> Int in
            return sum + Int(a == b)
        })
        
        if occurrences < 2 {
            subset.append(a)
        }
        
    }
    
    return subset
}