111. Minimum Depth of Binary Tree

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一看就是BFS，搜到left/right child 都是 null 的时候就返回当前深度。

class Solution:
    def minDepth(self, root: TreeNode) -> int:
        if not root: return 0
        res = 0
        queue = collections.deque([root])
        while queue:
            res += 1
            n_ = len(queue)
            for i in range(n_):
                node = queue.popleft()
                if not node.left and not node.right:
                    return res
                if node.left: queue.append(node.left)
                if node.right: queue.append(node.right)