无法转换List< List< int>>.返回类型IList< IList< int>> [英] Unable to convert List<List<int>> to return type IList<IList<int>>

问题描述

对于级别顺序遍历，为什么会发生此异常? 发生以下异常:

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

无法将类型'System.Collections.Generic.List<System.Collections.Generic.List<int>>'隐式转换为'System.Collections.Generic.IList<System.Collections.Generic.IList<int>>'.存在显式转换(您是否缺少演员表?)

Cannot implicitly convert type 'System.Collections.Generic.List<System.Collections.Generic.List<int>>' to 'System.Collections.Generic.IList<System.Collections.Generic.IList<int>>'. An explicit conversion exists (are you missing a cast?)

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

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

    que.Enqueue(root);
    while(que.Count!=0)
    {
        int n = que.Count;
        var subList = new List<int>();
        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;
}

推荐答案

只需将结果的声明更改为List<IList<int>>.

Just change the declaration of your result to List<IList<int>>.

List<T>实现IList<T>，但是List<List<T>>不实现IList<IList<int>>.除非以这种方式定义，否则通用参数不是协变或协变的，而IList<T>则不是，因此类型必须完全匹配.

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

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

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

    que.Enqueue(root);
    while (que.Count != 0)
    {
        int n = que.Count;
        var subList = new List<int>();
        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;
}

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