Recursively call `search(TreeNode root, int length, TreeNode parent)`

to get result.

```
public int longestConsecutive(TreeNode root) {
return search(root, 0, null);
}
private int search(TreeNode root, int length, TreeNode parent) {
if (root == null) { return length; }
if (parent == null || parent.val + 1 != root.val) {
length = 0;
}
length += 1;
int left = search(root.left, length, root);
int right = search(root.right, length, root);
return Math.max(length, Math.max(left, right));
}
```