leetcode104，111，543 求二叉树深度的dfs解法

class Solution:
    def maxDepth(self, root: TreeNode) -> int:
        if root == None:
            return 0
        else:
            leftdepth = self.maxDepth(root.left)
            rightdepth = self.maxDepth(root.right)
            return max(leftdepth, rightdepth) + 1 #！！！关键在于+1

class Solution:
    def minDepth(self, root: TreeNode) -> int:
        if not root:
            return 0
        if not root.left:
            return self.minDepth(root.right)+1
        if not root.right:
            return self.minDepth(root.left)+1
        return min(self.minDepth(root.left), self.minDepth(root.right))+1

class Solution:
    def diameterOfBinaryTree(self, root: TreeNode) -> int:
        self.ans = 1
        def depth(node):
            if not node: return 0
            L = depth(node.left)
            R = depth(node.right)
            self.ans = max(self.ans, L+R+1)
            return max(L, R) + 1
        depth(root)
        return self.ans - 1