### 题目

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.

### 递归

#### 代码

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
class Solution {

private static int diameter = 0;

public int diameterOfBinaryTree(TreeNode root) {
diameter = 0;
depth(root);
return diameter;
}
private int depth(TreeNode root) {
if (root == null) { return -1; }
int left = depth(root.left) + 1;
int right = depth(root.right) + 1;
diameter = Math.max(diameter, (left + right)); // 左，右子树的最大深度相加
return Math.max(left,right);
}
}