定义：从根节点到最近叶子结点的最短路径上的节点个数

实现：

class TreeNode(object):
    def __init__(self, item):
        self.item = item
        self.left = None
        self.right = None


def create_tree():
    n1 = TreeNode(1)
    n2 = TreeNode(2)
    n3 = TreeNode(3)
    n4 = TreeNode(4)
    n5 = TreeNode(5)
    n6 = TreeNode(6)
    n7 = TreeNode(7)
    n1.left = n2
    n1.right = n3
    n2.left = n4
    n2.right = n5
    n4.left = n6
    n4.right = n7
    return n1


def min_deep(head):
    if not head:
        # 空树返回0；当前没有数据，迭代停止条件
        return 0
    if not head.left:
        # 没有左孩子，计算右孩子
        return min_deep(head.right) + 1
    if not head.right:
        # 没有右孩子，计算左孩子
        return min_deep(head.left) + 1
    # 左右孩子完整，取最小的
    return min(min_deep(head.left), min_deep(head.right)) + 1


if __name__ == '__main__':
    print(min_deep(create_tree()))

结果返回：2