687. Longest Univalue Path

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int max = 0;
    public int longestUnivaluePath(TreeNode root) {
        helper(root);
        return max;
    }
    
    private int helper(TreeNode root) {
        if (root == null) {
            return 0;
        }
        int left = helper(root.left);
        int right = helper(root.right);
        left = (root.left != null && root.left.val == root.val) ? left + 1 : 0;
        right = (root.right != null && root.right.val == root.val) ? right + 1 : 0;
        max = Math.max(max, left + right);
        return Math.max(left, right);
    }
}