LowestCommonAncestor.swift

func lowestCommonAncestor<ID: Hashable>(of ids: (ID, ID), parent: (ID) -> ID?) -> ID? {
    var ancestor: (ID?, ID?) = ids
    var visited: (Set, Set) = ([ids.0], [ids.1])
    
    while ancestor.0 != nil || ancestor.1 != nil {
        if let ancestor = ancestor.0 {
            guard !visited.1.contains(ancestor) else { return ancestor }
        }
        if let ancestor = ancestor.1 {
            guard !visited.0.contains(ancestor) else { return ancestor }
        }

        if let next = ancestor.0.flatMap(parent) {
            ancestor.0 = next
            visited.0.insert(next)
        }
        if let next = ancestor.1.flatMap(parent) {
            ancestor.1 = next
            visited.1.insert(next)
        }
    }
    return nil
}