minimum depth of binary tree

# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class (object):
    def minDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if not root:
            return 0
        if not root.left and not root.right:
            return 1
        left = self.minDepth(root.left)
        right = self.minDepth(root.right)
        if not root.left:
            return right + 1
        if not root.right:
            return left + 1
        return min(left, right) + 1