[Swift]LeetCode298. 二叉樹最長連續序列 $ Binary Tree Longest Consecutive Sequence

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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.

---

給定二叉樹，求最長連續序列路徑的長度。

路徑是指沿着父子鏈接從某個起始節點到樹中任何節點的任何節點序列。最長的連續路徑必須是從父路徑到子路徑（不能是反向路徑）。

例如，

   1
    \
     3
    / \
   2   4
        \
         5

最長的連續序列路徑是3-4-5，因此返回3。

   2
    \
     3
    / 
   2    
  / 
 1

最長的連續序列路徑是2-3，而不是3-2-1，因此返回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 }