Leetcode 298 Binary Tree Longest Consecutive Sequence

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.
Solution 1: Simple DFS and keep tracking the consecutive sequence will solve it.

 public class Solution {  
   public int longestConsecutive(TreeNode root) {  
     if (root==null) return 0;  
     int[] res=new int[1];  
     dfs(root,1,res);  
     return res[0];  
   }  
   private void dfs(TreeNode x, int len, int[] res) {  
     res[0]=Math.max(res[0],len);  
     if (x.left!=null) {  
       if (x.left.val==x.val+1) dfs(x.left,len+1,res);  
       else dfs(x.left,1,res);  
     }  
     if (x.right!=null) {  
       if (x.right.val==x.val+1) dfs(x.right,len+1,res);  
       else dfs(x.right,1,res);  
     }  
   }  
 }