Returning all duplicates from the array in swift.

Duplicates.swift

//Duplicate
func returnDuplicates(arr: [Int]) -> [Int] {
    var dict: [Int: Bool] = [:]
    var duplicates: [Int] = []
    for i in arr {
        if dict.keys.contains(i) {
            duplicates.append(i)
        } else {
            dict[i] = true
        }
    }
    return duplicates
}

returnDuplicates(arr: [1,2,3,5,1,3,2,7]) //result [1, 3, 2]