https://leetcode.com/problems/diameter-of-binary-tree/#/description
Given a binary tree, you need to compute the length of the diameter of the tree. The diameter of a binary tree is the length of the longest path between any two nodes in a tree. This path may or may not pass through the root.
Example:
Given a binary tree
1
/ \
2 3
/ \
4 5
Return 3, which is the length of the path [4,2,1,3] or [5,2,1,3].
Note: The length of path between two nodes is represented by the number of edges between them.
Hint:
Answer = max (max left_depth + max_right_depth, max left_depth, max right_depth)
The "max left_depth + max_right_depth" holds true when the right and left subtrees exist, and "max left_depth" holds true when only left subtree exists; likwise, "max right_depth" holds true when only right subtree exists.
Back-up knowledge:
Q： How to calculate the depth of tree?
A:
1) Traverse the depth of left subtree.
2) Traverse the depth of right subtree.
3) Compare the two depths.
If left_depth > right_depth, then return left_depth + 1.
else left_depth < right_depth, then return right_depth + 1.
P.S. The reason why we + 1 is that the root is at level 1.
class Solution:
def TreeDepth(self, node):
if node == None:
return 0
left_depth = Solution.TreeDepth(self, node.left)
right_depth = Solution.TreeDepth(self, node.right)
return max(left_depth, right_depth) + 1
Note:
1 It is implemented in a recursive manner.
Sol:
# 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 diameterOfBinaryTree(self, root):
"""
:type root: TreeNode
:rtype: int
"""
self.max_len = 0
def depth(node):
if not node:
return 0
left_depth = depth(node.left)
right_depth = depth(node.right)
self.max_len = max(self.max_len, left_depth + right_depth)
return max(left_depth, right_depth) + 1
depth(root)
return self.max_len
Note:
1 Implant the idea of recursion in your head. Don't think of "trace back".
2 self.max_len is a customer-defined variable/method. It's like "global variable", otherwise max_len can not carry the value in def depth to def diameterOfBinaryTree.