LeetCode104 Maximum Depth of Binary Tree

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.

<思路>用递归的方法，如果有左子树，L+1；如果有右子树，R+1。最后返回深度。

# 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):
        """
        :type root: TreeNode
        :rtype: int
        """
        if root is None:
            return 0
        l = 1 + self.maxDepth(root.left)     #left child tree depth
        r = 1 + self.maxDepth(root.right)    #right child tree depth
        
        return max(l,r)

discuss里的BFS算法，学习借鉴。每次queue里存储一层的节点。

class Solution(object):
    def maxDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        depth = 0
        level = [root] if root else []
        while level:
            depth += 1
            queue = []
            for el in level:
                if el.left:
                    queue.append(el.left)
                if el.right:
                    queue.append(el.right)
            level = queue
            
        return depth