Unable to convert List> to return type IList>

Question

For level order traversal why does this exception occur? Following exception occurs:

Cannot implicitly convert type \'System.Collections.Generic

Answer 1

Just change the declaration of your result to List>.

List implements IList, but List> does not implement IList>. Generic parameters are not covariant or contravariant unless defined that way and IList is not, so the type must match exactly.

public IList> LevelOrder(TreeNode root)
{
    var result = new List>();
    var que = new Queue();

    //if(root==null) return result;

    que.Enqueue(root);
    while (que.Count != 0)
    {
        int n = que.Count;
        var subList = new List();
        for (int i = 0; i < n; i++)
        {
            if (que.Peek().left != null)
                que.Enqueue(que.Peek().left);
            if (que.Peek().right != null)
                que.Enqueue(que.Peek().right);
            subList.Add(que.Dequeue().val);
        }
        result.Add(subList);
    }
    return result;
}