private int path;

public int longestUnivaluePath(TreeNode root) {

dfs(root);

return path;

}

private int dfs(TreeNode root) {

if (root == null) return 0;

int left = dfs(root.left);

int right = dfs(root.right);

int leftPath = ( root.left != null && root.left.val == root.val ) ? left + 1 : 0;

int rightPath = ( root.right != null && root.right.val == root.val) ? right + 1 : 0;

path = Math.max(leftPath + rightPath, path);

return Math.max(leftPath, rightPath);

}

class TreeNode {

int val;

TreeNode left;

TreeNode right;

TreeNode(int x) { val = x; }

}