方法一：递归 
class Solution:
    def TreeDepth(self , pRoot: TreeNode) -> int:
        # write code here
        if not pRoot:
            return 0
        left = right =0
        if pRoot.left:
            left = self.TreeDepth(pRoot.left)
        if pRoot.right:
            right = self.TreeDepth(pRoot.right)
        return max([left,right])+1
方法二:层次遍历，每经历一层， count+=1 
class Solution:
    def TreeDepth(self , pRoot: TreeNode) -> int:
        # write code here
        if pRoot is None:
            return 0
        count = 0
        now_layer =[pRoot]
        next_layer = []
        while now_layer:
            for i in now_layer:
                if i.left:
                    next_layer.append(i.left)
                if i.right:
                    next_layer.append(i.right)
            count +=1
            now_layer, next_layer = next_layer,[]
        return count