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# Diameter of Binary Tree(递归含条件判断）

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.

分析

递归返回depth,但是同时在递归里更新全局变量diameter，不返回而已。这里把getDepth函数作为内置函数

注意内置函数写法，里面不含任何self，除了使用的全局变量。

```
# 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.d = 0

        def depth(root):
            if not root:
                return 0
            l,r = depth(root.left),depth(root.right)
            self.d = max(self.d,l+r)
            return 1+ max(l,r)

        depth(root)
        return self.d
```