Binary Tree Longest Consecutive Seq

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.

    private int max = 1;
    public int longestConsecutive(TreeNode root){
        Helper(root);
        return max;
    }
    public int Helper(TreeNode root) {
        if(root == null) return 0;
        else if(root.left == null && root.right == null){
            return 1;
        }
        else{
            int left = Helper(root.left), right = Helper(root.right);
            if(root.left != null && root.val != root.left.val - 1){
                left = 0;
            }
            if(root.right != null && root.val != root.right.val - 1){
                right = 0;
            }
            int curMax = Math.max(left + 1, right + 1);
            max = Math.max(max, curMax);
            return curMax;
        }
    }