111.二叉树的最小深度

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def minDepth(self, root: TreeNode) -> int:
        if root is None:
            return 0
        if root.left and root.right:
            return min(self.minDepth(root.left), self.minDepth(root.right)) + 1
        #   3
        #  / \
        # 9
        # 返回的应该是2，而不是1
        else:
            return max(self.minDepth(root.left), self.minDepth(root.right)) + 1