298. Binary Tree Longest Consecutive Sequence

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int longestConsecutive(TreeNode root) {
        if(root == null) return 0;
        helper(root);
        return re;
    }

    int helper(TreeNode root)
    {
        if(root == null) return 0;
        int left = helper(root.left), right = helper(root.right);
        int ans = 1; // for root
        if(root.left != null && root.left.val == root.val + 1)
        {
            ans += left;
        }
        if(root.right != null && root.right.val == root.val + 1)
        {
            ans = Math.max(ans, 1 + right);
        }
        re = Math.max(re, ans);
        return ans;
    }

    int re = 0;

}