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.

Note: A leaf is a node with no children.

Example 1:

Input: root = [3,9,20,null,null,15,7] Output: 2 Example 2:

Input: root = [2,null,3,null,4,null,5,null,6] Output: 5

My solution：

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def minDepth(self, root):
        """
        :type root: Optional[TreeNode]
        :rtype: int
        """
        if not root:
            return 0
        depth = 0
        queue = deque()
        queue.append(root)
        while queue:
            length = len(queue)
            depth += 1
            for i in range(length):
                cur = queue.popleft()
                if not cur.left and not cur.right:
                    return depth
                if cur.left:
                    queue.append(cur.left)
                if cur.right:
                    queue.append(cur.right)
        return depth