C#•2y ago

✅ The code only add root nodes into Queue instead of the whole pre-defined int arrays

I just got stuck with this puzzle https://leetcode.com/problems/binary-tree-level-order-traversal/solutions/3100951/easy-and-clean-solution/ My code is right here:

`public IList<IList<int>> LevelOrder(TreeNode root) {
        List<IList<int>> res = new List<IList<int>>();
        if(root == null){
            return res;
        }

        Queue<int> q = new Queue<int>();
        List<int> separation = new List<int>();
        q.Enqueue(root.val);
        while(q.Count > 0){
            
            separation = new List<int>();
            for(int i = 0; i < q.Count; i++){
                TreeNode curr = new TreeNode();
                curr.val = q.Dequeue();
                
                separation.Add(curr.val);
                if(curr.left != null){
                    q.Enqueue(curr.left.val);
                }

                if(curr.right != null){
                    q.Enqueue(curr.right.val);
                }   
            }
            res.Add(separation);
        }
        return res;
    }
}

`public IList<IList<int>> LevelOrder(TreeNode root) {
        List<IList<int>> res = new List<IList<int>>();
        if(root == null){
            return res;
        }

        Queue<int> q = new Queue<int>();
        List<int> separation = new List<int>();
        q.Enqueue(root.val);
        while(q.Count > 0){
            
            separation = new List<int>();
            for(int i = 0; i < q.Count; i++){
                TreeNode curr = new TreeNode();
                curr.val = q.Dequeue();
                
                separation.Add(curr.val);
                if(curr.left != null){
                    q.Enqueue(curr.left.val);
                }

                if(curr.right != null){
                    q.Enqueue(curr.right.val);
                }   
            }
            res.Add(separation);
        }
        return res;
    }
}

` But the result is still not complete. The code only adds any root node into the queue/list while it was supposed to print out the binary tree with levels. (see the attached screenshot). Could anyone kindly point me in the right direction?

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5 Replies

Florian Voß•2y ago

the issue is curr.left and curr.right will always be null in your code because your curr node is not the root node but a new node that you created which doesn't have any children your curr node should be the one that you dequeue from the queue, not a new node. i suggest storing the Nodes in the qeueue with a Queue<TreeNode> instead of their integer values. Then make curr be the dequeued node and you can access its children you only copied the integer value of the root node into your own node that you created, that doesn't make it hold the same child nodes you understand @morry329 ? you could ofc also copy over the child nodes to your newly created node but you better don't create new nodes at all here

Accord•2y ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

morry329#OP•2y ago

Thank you so much for your amazing explanation! One more small (trivial) question. So storing the nodes as a Queue<TreeNode> makes sense for this problem as we need to check if the child "node" has its own child elements? As I check other LC answers I have noticed List<int> as each level of child elements and in the end those answers return the List<IList<int>>, not List<IList<TreeNode>>. You see, I still have some gaps in understanding

Florian Voß•2y ago

you return List<IList<int>> because thats the method signature / return type LeetCode has given to you. however i personally think a Queue<TreeNode> is best suited to get to that result.

List<IList<int>> result = new();
Queue<TreeNode> nodes = new();
while(nodes.Any()){
  //dequeue node from nodes and add it's value to result
}

List<IList<int>> result = new();
Queue<TreeNode> nodes = new();
while(nodes.Any()){
  //dequeue node from nodes and add it's value to result
}

So storing the nodes as a Queue<TreeNode> makes sense for this problem as we need to check if the child "node" has its own child elements

exactly, we wanna check the left and right fields of the type, so we gotta store something of that type reference. Storing an integer makes it so that we cant use it to access the other ones

morry329#OP•2y ago

Ahhhh I got it 🙂 I can see this puzzle very clearly now XD Thank you so much for answering my questions a couple of times, that was a massive help

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✅ The code only add root nodes into Queue instead of the whole pre-defined int arrays