104-Maximum Depth of Binary Tree

100-Same Tree | Links:

题意

Given a binary tree, find its maximum depth. The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node. Note: A leaf is a node with no children.

Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7
return its depth = 3.

思路

• 树 postOrder
• 分别找左/右最大深度，取最大值 +1

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def maxDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if not root: return 0
        left = self.maxDepth(root.left)
        right = self.maxDepth(root.right)
        return max(left, right) +  1

分析:

• Time: O(n)
• Space: O(n)