Leetcode #947: Most Stones Removed with Same Row or Column

func removeStones(_ stones: [[Int]]) -> Int {
    var xDict = [Int: [Int]]()
    var yDict = [Int: [Int]]()
    for stone in stones {
        xDict[stone[0], default:[]].append(stone[1])
        yDict[stone[1], default:[]].append(stone[0])
    }
    var visited = Set<[Int]>()
    var answer = 0
    for stone in stones {
        var count = 0
        if !visited.contains([stone[0], stone[1]]) {
            var xArr = [stone[0]]
            var yArr = [stone[1]]
            while !xArr.isEmpty || !yArr.isEmpty {
                if !xArr.isEmpty {
                    let x = xArr.removeFirst()
                    let ar = xDict[x]!
                    for num in ar {
                        if !visited.contains([x, num]) {
                            yArr.append(num)
                            count += 1
                            visited.insert([x,num])
                        }
                    }
                }
                if !yArr.isEmpty {
                    let y = yArr.removeFirst()
                    let ar = yDict[y]!
                    for num in ar {
                        if !visited.contains([num, y]) {
                            xArr.append(num)
                            count += 1
                            visited.insert([num,y])
                        }
                    }
                }
            }
            count -= 1
        }
        answer += count
    }
    return answer
}