Minimum Depth of Binary Tree – returns None

I am working on LeetCode problem 111. Minimum Depth of Binary Tree:

Given a binary tree, find its minimum depth.

The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.

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Note: A leaf is a node with no children.

I have used a breadth first algorithm and tried to change it to align it to the problem. But the function is returning None.

Could anyone explain why that happens?

def minDepth(self, root: Optional[TreeNode]) -> int:
        queue=[]
        if root==None:
            return 0
        level=1
        while(len(queue)>0):
            n=len(queue)
            for i in range(n):
                node=queue.pop(0)
                if not node.right and not node.left:
                    return(level)
                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)
            level+=1

>Solution :

The problem is that your queue is empty at the moment the loop is about to start.

You should place the root node in it:

        if root is None:
            return 0
        queue = [root]  # <----
        level = 1
        while queue:  
            # ...etc