递归 function 返回 false 而不是 true

Question

private bool AnyChildDomainExists(Domain parentDomain, int childDomainID)
{
    if (parentDomain.DomainID == childDomainID)
        return true;

    foreach(Domain domain in parentDomain.Domains)
    {
        return AnyChildDomainExists(domain, childDomainID);
    }

    return false;
}

For example this is my tree:

Root
  Child 1
     GrandChild 1
  Child 2
     GrandChild 2

I pass in the function AnyChildDomainExists(root, GrandChild 2'sID) it returns false. There is a small issue in the function but I am unable to figure it out.

Answer 1

private bool AnyChildDomainExists(Domain parentDomain, int childDomainID)
{
    if (parentDomain.DomainID == childDomainID)
        return true;

    return parentDomain.Domains.Any(domain => AnyChildDomainExists(domain, childDomainID));

}

Check if any of the child domain Ids match, using LINQ Any .

Answer 2

Another way to do this is to only return from the inner loop if the result is true . As it stands, you're returning a value on the first iteration of child domains, which is not what you want.

For example:

private bool AnyChildDomainExists(Domain parentDomain, int childDomainID)
{
    if (parentDomain.DomainID == childDomainID) return true;

    foreach(Domain domain in parentDomain.Domains)
    {
        if (AnyChildDomainExists(domain, childDomainID)) return true;
    }

    return false;
}

Answer 3

The way the loop is written, it returns the result from the first leaf node immediately and propagates it all the way to the top. This means that if GrandChild 2 returns false .

One way to fix this is to OR the results of all children in the loop, eg:

private bool AnyChildDomainExists(Domain parentDomain, int childDomainID)
{
    if (parentDomain.DomainID == childDomainID)
        return true;

    bool result=false;
    foreach(Domain domain in parentDomain.Domains)
    {
        result|=AnyChildDomainExists(domain, childDomainID);
    }

    return result;
}

This code will return true for node 22 (GrandChild 2):

    var tree=new Domain(0,new[]{
        new Domain(1,new[]{
            new Domain(11,new Domain[0])
        }),
        new Domain(2,new[]{
            new Domain(22,new Domain[0])
        })
    });

    var found=AnyChildDomainExists(tree,22);
    Debug.Assert(found);