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Given a binary tree, find the length of the longest consecutive sequence path.

 The path refers to any sequence of nodes from some starting node to any node in the tree along the parent-child connections. The longest consecutive path need to be from parent to child (cannot be the reverse).

For example,

   1
    \
     3
    / \
   2   4
        \
         5

Longest consecutive sequence path is 3-4-5, so return 3.

   2
    \
     3
    / 
   2    
  / 
 1

Longest consecutive sequence path is 2-3,not3-2-1, so return 2.

给定二叉树，求最长连续序列路径的长度。

路径是指沿着父子连接从某个起始节点到树中任何节点的任何节点序列。最长的连续路径必须是从父路径到子路径（不能是反向路径）。

例如，

   1
    \
     3
    / \
   2   4
        \
         5

最长的连续序列路径是3-4-5，所以返回3。

   2
    \
     3
    / 
   2    
  / 
 1

最长的连续序列路径是2-3，而不是3-2-1，所以返回2。

Solution:

 1 public class TreeNode {
 2     public var val: Int
 3     public var left: TreeNode?
 4     public var right: TreeNode?
 5     public init(_ val: Int) {
 6         self.val = val
 7         self.left = nil
 8         self.right = nil
 9     }
10 }
11 
12 class Solution {
13     func longestConsecutive(_ root: TreeNode?) -> Int {
14         var num:Int = 0
15         return helper(root,nil,num)
16     }
17     
18     func helper(_ root: TreeNode?,_ p: TreeNode?,_ res:Int) -> Int
19     {
20         var res = res
21         if root == nil {return res}
22         res = (p != nil && root!.val == p!.val + 1) ? res + 1 : 1
23         return max(res, max(helper(root?.left, root, res), helper(root?.right, root, res)))
24     }
25 }