## 题目描述

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.

## 解法一

### Python

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

class Solution:
def diameterOfBinaryTree(self, root):
"""
:type root: TreeNode
:rtype: int
"""
self.result = -1

def diameter(root):
if root is None:
return 0
left = diameter(root.left)
right = diameter(root.right)
length = left + right + 1
self.result = max(self.result, length)
return max(left, right) + 1

diameter(root)

return 0 if self.result == -1 else self.result - 1``````

### Java

``````/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public int diameterOfBinaryTree(TreeNode root) {
class Utils {
int result = -1;

int diameter(TreeNode root) {
if (root == null) {
return 0;
}
int left = diameter(root.left);
int right = diameter(root.right);
int length = left + right + 1;
result = Math.max(result, length);
return Math.max(left, right) + 1;
}
}
Utils utils = new Utils();
utils.diameter(root);
return utils.result == -1 ? 0 : utils.result - 1;
}
}``````

### C++

``````/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
int diameterOfBinaryTree(TreeNode *root) {
class Utils {
public:
int result = -1;

int diameter(TreeNode *root) {
if (root == nullptr) {
return 0;
}
int left = diameter(root->left);
int right = diameter(root->right);
int length = left + right + 1;
result = max(result, length);
return max(left, right) + 1;
}
};
Utils utils;
utils.diameter(root);
return utils.result == -1 ? 0 : utils.result - 1;
}
};``````