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.
解题要点：
1.利用一个stack放置从树中遍历出来的节点，设置一个指针，遇到每层最后一个往后移，层数加一；
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution:
def maxDepth(self, root):
"""
:type root: TreeNode
:rtype: int
"""
ans = 0
if root == None:
return ans
st = []
st.append(root)
# temp, gp = st[0], st[0]
gp = st[0]
ans += 1
while len(st) != 0:
temp = st[0]
if temp.left != None:
st.append(temp.left)
if temp.right != None:
st.append(temp.right)
if gp == temp:
gp = st[len(st)-1]
if gp != st[0]:
ans += 1
st.pop(0)
return ans还有一种是递归的方法，代码比较短，不用想前面多用一个容器；
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution:
def maxDepth(self, root):
"""
:type root: TreeNode
:rtype: int
"""
if root == None:
return 0
else:
return max(self.maxDepth(root.left),self.maxDepth(root.right))+1