687. Longest Univalue Path 687.最长单值路径

class Solution {
    int length = 0;
    
    public int longestUnivaluePath(TreeNode root) {
        //cc
        if (root == null)
            return 0;
        
        helper(root, root.val);
        
        return length;
    }
    
    public int helper(TreeNode node, int value) {
        //cc
        if (node == null) 
            return 0;
        
        int left = helper(node.left, node.val);
        int right = helper(node.right, node.val);
        
        length = Math.max(length, left + right);
        
        if (node.val == value)
            return Math.max(left, right) + 1;
        
        return 0;
    }
}