## 題解

### DFS (Recursion)

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def maxDepth(self, root: Optional[TreeNode]) -> int:
        self.max_depth = 0
        def dfs(node: TreeNode,level: int):
            if node:
                self.max_depth = max(self.max_depth, level)
                if node.left:
                    dfs(node.left, level + 1)
                if node.right:
                    dfs(node.right, level + 1)
        
        dfs(root,1)

        return self.max_depth

### BFS

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def maxDepth(self, root: Optional[TreeNode]) -> int:
        queue = [root]
        max_depth = 0

        while queue:
            length = len(queue)
            state = False
            for i in range(length):
                node = queue.pop(0)
                if node:
                    state = True
                    if node.left:
                        queue.append(node.left)
                    if node.right:
                        queue.append(node.right)
            if state:
                max_depth += 1
        return max_depth