104. Maximum Depth of Binary Tree

Question 238

https://leetcode.com/problems/maximum-depth-of-binary-tree/description/

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.

Example:

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

    3
   / \
  9  20
    /  \
   15   7

return its depth = 3.

Answer

v1.stack

將 tree 一層一層的輸進 stack 中，有幾層就會有幾個 while (第一個 while)，每一層的點會依照 .left / .right 的方式輸入進 stack 中（第二個 while ）

v2. recursive (92.50%)

v1

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
def maxDepth(self, root):     
    if not root:
     return 0

    tmpstack = [root]
    # for count depth
    num = 0

    while tmpstack:
        nextL = []
    
        while tmpstack:
            top = tmpstack.pop()
            if top.left:
                nextL.append(top.left)
            if top.right:
                nextL.append(top.right)
                
        tmpstack = nextL
        num += 1
    return num

v2

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
 def maxDepth(self, root):     
    
    if not root:
        return 0
    
    return max(self.maxDepth(root.left), self.maxDepth(root.right)) + 1